store SkAlphaType inside SkBitmap, on road to support unpremul

BUG=
R=bsalomon@google.com, scroggo@google.com

Review URL: https://codereview.chromium.org/25275004

git-svn-id: http://skia.googlecode.com/svn/trunk@11877 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/images/SkImageDecoder_libwebp.cpp b/src/images/SkImageDecoder_libwebp.cpp
index 8bb716b..68014dd 100644
--- a/src/images/SkImageDecoder_libwebp.cpp
+++ b/src/images/SkImageDecoder_libwebp.cpp
@@ -293,11 +293,8 @@
         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,