make info real in SkPixelRef, and add bitmap::asImageInfo

BUG=
R=scroggo@google.com

Review URL: https://codereview.chromium.org/108663004

git-svn-id: http://skia.googlecode.com/svn/trunk@12586 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/images/SkDecodingImageGenerator.cpp b/src/images/SkDecodingImageGenerator.cpp
index 2b80444..e49f9a4 100644
--- a/src/images/SkDecodingImageGenerator.cpp
+++ b/src/images/SkDecodingImageGenerator.cpp
@@ -119,11 +119,9 @@
     if (bitmap.config() == SkBitmap::kNo_Config) {
         return false;
     }
-    if (!SkBitmapToImageInfo(bitmap, &fInfo)) {
+    if (!bitmap.asImageInfo(&fInfo)) {
         // We can't use bitmap.config() as is.
-        // Must be kARGB_4444_Config.
         if (!bitmap.canCopyTo(SkBitmap::kARGB_8888_Config)) {
-            // kARGB_4444_Config can copy to kARGB_8888.
             SkDEBUGFAIL("!bitmap->canCopyTo(SkBitmap::kARGB_8888_Config)");
             return false;
         }