Optimize CSS box-shadow performance by caching the SkMask of the blur effect.

src/core/SkScaledImageCache.cpp

Index: src/core/SkScaledImageCache.cpp
diff --git a/src/core/SkScaledImageCache.cpp b/src/core/SkScaledImageCache.cpp
index a6bffd899fe66938a91fccf1dc90660d5d7e9a51..27b7ea684e75877b37098c01400a00fa168f1bad 100644
--- a/src/core/SkScaledImageCache.cpp
+++ b/src/core/SkScaledImageCache.cpp
@@ -276,7 +276,8 @@ bool SkScaledImageCacheDiscardableAllocator::allocPixelRef(SkBitmap* bitmap,
     }
 
     // can we relax this?
-    if (kN32_SkColorType != bitmap->colorType()) {
+    if (kN32_SkColorType != bitmap->colorType()
+        && kAlpha_8_SkColorType != bitmap->colorType()) {
         return false;
     }