Revert of Revert of replace kIgnore_SkAlphaType with kUnknown_SkAlphaType (patchset #1 id:1 of https://codereview.chromium.org/966753002/)

Reason for revert:
Android has been updated, so we can re-land this.

Original issue's description:
> Revert of replace kIgnore_SkAlphaType with kUnknown_SkAlphaType (patchset #3 id:40001 of https://codereview.chromium.org/964613002/)
>
> Reason for revert:
> This breaks Android framework build. See
>
> https://android-build.storage.googleapis.com/builds/git_master-skia-linux-volantisg-userdebug/1759130/7f0fcdbf3c0d47530d770a15c912298dac20dd7d90a01de63ecb9ba16c96b25c/logs/build_error.log?Expires=1425047630&GoogleAccessId=701025073339-mqn0q2nvir9iurm6q5d00tdv7blbgvjr%40developer.gserviceaccount.com&Signature=BHPS1lffyH7WykwyEw5RXVVXWqQK5rab%2B6Pl1VaUrTlmWT%2FXb6fq%2FBe22AUDZYFPG24MBR%2F2IzIDzTqH3mihQS9wIkcKJ4kJpA2G%2BI3l8jCX20brZSr3bsDSx%2F4wsLwU1bBpIxI%2FbrauoPRqddbXP8sNI1l51u6ZCxpvRQms9Mc%3D
>
> We need to stop them from using kIgnore before we can reland this.
>
> Original issue's description:
> > replace kIgnore_SkAlphaType with kUnknown_SkAlphaType
> >
> > BUG=skia:
> > TBR=
> >
> > Committed: https://skia.googlesource.com/skia/+/1c8aeeaebce9675197be31bd769e8ffa2531bf86
>
> TBR=reed@google.com,reed@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=skia:
>
> Committed: https://skia.googlesource.com/skia/+/574290f61a47bd1ce1f9e2d941533bda9c8f03fe

TBR=reed@chromium.org,scroggo@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review URL: https://codereview.chromium.org/966853002
diff --git a/src/core/SkBitmap.cpp b/src/core/SkBitmap.cpp
index e77bbbd..0b586cb 100644
--- a/src/core/SkBitmap.cpp
+++ b/src/core/SkBitmap.cpp
@@ -182,8 +182,8 @@
             SkASSERT(fInfo.height() <= prInfo.height());
             SkASSERT(fInfo.colorType() == prInfo.colorType());
             switch (prInfo.alphaType()) {
-                case kIgnore_SkAlphaType:
-                    SkASSERT(fInfo.alphaType() == kIgnore_SkAlphaType);
+                case kUnknown_SkAlphaType:
+                    SkASSERT(fInfo.alphaType() == kUnknown_SkAlphaType);
                     break;
                 case kOpaque_SkAlphaType:
                 case kPremul_SkAlphaType:
diff --git a/src/core/SkBitmapDevice.cpp b/src/core/SkBitmapDevice.cpp
index e545bae..37cbff5 100644
--- a/src/core/SkBitmapDevice.cpp
+++ b/src/core/SkBitmapDevice.cpp
@@ -25,7 +25,7 @@
     // TODO: can we stop supporting kUnknown in SkBitmkapDevice?
     if (kUnknown_SkColorType == info.colorType()) {
         if (newAlphaType) {
-            *newAlphaType = kIgnore_SkAlphaType;
+            *newAlphaType = kUnknown_SkAlphaType;
         }
         return true;
     }
diff --git a/src/core/SkConfig8888.cpp b/src/core/SkConfig8888.cpp
index 8b47a20..85e208f 100644
--- a/src/core/SkConfig8888.cpp
+++ b/src/core/SkConfig8888.cpp
@@ -54,8 +54,8 @@
 }
 
 static AlphaVerb compute_AlphaVerb(SkAlphaType src, SkAlphaType dst) {
-    SkASSERT(kIgnore_SkAlphaType != src);
-    SkASSERT(kIgnore_SkAlphaType != dst);
+    SkASSERT(kUnknown_SkAlphaType != src);
+    SkASSERT(kUnknown_SkAlphaType != dst);
 
     if (kOpaque_SkAlphaType == src || kOpaque_SkAlphaType == dst || src == dst) {
         return kNothing_AlphaVerb;
diff --git a/src/core/SkImageInfo.cpp b/src/core/SkImageInfo.cpp
index 146a3ae..0e8b0b1 100644
--- a/src/core/SkImageInfo.cpp
+++ b/src/core/SkImageInfo.cpp
@@ -50,7 +50,7 @@
                                   SkAlphaType* canonical) {
     switch (colorType) {
         case kUnknown_SkColorType:
-            alphaType = kIgnore_SkAlphaType;
+            alphaType = kUnknown_SkAlphaType;
             break;
         case kAlpha_8_SkColorType:
             if (kUnpremul_SkAlphaType == alphaType) {
@@ -61,7 +61,7 @@
         case kARGB_4444_SkColorType:
         case kRGBA_8888_SkColorType:
         case kBGRA_8888_SkColorType:
-            if (kIgnore_SkAlphaType == alphaType) {
+            if (kUnknown_SkAlphaType == alphaType) {
                 return false;
             }
             break;
diff --git a/src/utils/mac/SkCreateCGImageRef.cpp b/src/utils/mac/SkCreateCGImageRef.cpp
index 51a5ed9..4675d71 100644
--- a/src/utils/mac/SkCreateCGImageRef.cpp
+++ b/src/utils/mac/SkCreateCGImageRef.cpp
@@ -12,8 +12,9 @@
 static CGBitmapInfo ComputeCGAlphaInfo_RGBA(SkAlphaType at) {
     CGBitmapInfo info = kCGBitmapByteOrder32Big;
     switch (at) {
+        case kUnknown_SkAlphaType:
+            break;
         case kOpaque_SkAlphaType:
-        case kIgnore_SkAlphaType:
             info |= kCGImageAlphaNoneSkipLast;
             break;
         case kPremul_SkAlphaType:
@@ -29,8 +30,9 @@
 static CGBitmapInfo ComputeCGAlphaInfo_BGRA(SkAlphaType at) {
     CGBitmapInfo info = kCGBitmapByteOrder32Little;
     switch (at) {
+        case kUnknown_SkAlphaType:
+            break;
         case kOpaque_SkAlphaType:
-        case kIgnore_SkAlphaType:
             info |= kCGImageAlphaNoneSkipFirst;
             break;
         case kPremul_SkAlphaType: