store SkAlphaType inside SkBitmap, on road to support unpremul

src/images/SkImageDecoder_libwebp.cpp

Index: src/images/SkImageDecoder_libwebp.cpp
diff --git a/src/images/SkImageDecoder_libwebp.cpp b/src/images/SkImageDecoder_libwebp.cpp
index 8bb716bae82a40807c0a759a8fcf0bc22ba357fc..68014dd39247f0c663305584ad62ffde7ee978e7 100644
--- a/src/images/SkImageDecoder_libwebp.cpp
+++ b/src/images/SkImageDecoder_libwebp.cpp
@@ -293,11 +293,8 @@ bool SkWEBPImageDecoder::setDecodeConfig(SkBitmap* decodedBitmap,
         return false;
     }
 
-    decodedBitmap->setConfig(config, width, height, 0);
-
-    decodedBitmap->setIsOpaque(!fHasAlpha);
-
-    return true;
+    return decodedBitmap->setConfig(config, width, height, 0,
+                                    fHasAlpha ? kPremul_SkAlphaType : kOpaque_SkAlphaType);
 }
 
 bool SkWEBPImageDecoder::onBuildTileIndex(SkStreamRewindable* stream,