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/include/core/SkImageInfo.h b/include/core/SkImageInfo.h
index f928416..d915c09 100644
--- a/include/core/SkImageInfo.h
+++ b/include/core/SkImageInfo.h
@@ -19,12 +19,7 @@
  *  Describes how to interpret the alpha compoent of a pixel.
  */
 enum SkAlphaType {
-    /**
-     *  All pixels should be treated as opaque, regardless of the value stored
-     *  in their alpha field. Used for legacy images that wrote 0 or garbarge
-     *  in their alpha field, but intended the RGB to be treated as opaque.
-     */
-    kIgnore_SkAlphaType,
+    kUnknown_SkAlphaType,
 
     /**
      *  All pixels are stored as opaque. This differs slightly from kIgnore in
@@ -53,11 +48,7 @@
 };
 
 static inline bool SkAlphaTypeIsOpaque(SkAlphaType at) {
-    SK_COMPILE_ASSERT(kIgnore_SkAlphaType < kOpaque_SkAlphaType, bad_alphatype_order);
-    SK_COMPILE_ASSERT(kPremul_SkAlphaType > kOpaque_SkAlphaType, bad_alphatype_order);
-    SK_COMPILE_ASSERT(kUnpremul_SkAlphaType > kOpaque_SkAlphaType, bad_alphatype_order);
-
-    return (unsigned)at <= kOpaque_SkAlphaType;
+    return kOpaque_SkAlphaType == at;
 }
 
 static inline bool SkAlphaTypeIsValid(unsigned value) {
@@ -161,7 +152,7 @@
         : fWidth(0)
         , fHeight(0)
         , fColorType(kUnknown_SkColorType)
-        , fAlphaType(kIgnore_SkAlphaType)
+        , fAlphaType(kUnknown_SkAlphaType)
         , fProfileType(kLinear_SkColorProfileType)
     {}
 
@@ -200,7 +191,7 @@
     }
 
     static SkImageInfo MakeUnknown(int width, int height) {
-        return SkImageInfo(width, height, kUnknown_SkColorType, kIgnore_SkAlphaType,
+        return SkImageInfo(width, height, kUnknown_SkColorType, kUnknown_SkAlphaType,
                            kLinear_SkColorProfileType);
     }