[Aura] Support transparent webkit.

content/browser/renderer_host/backing_store_skia.cc

Index: content/browser/renderer_host/backing_store_skia.cc
diff --git a/content/browser/renderer_host/backing_store_skia.cc b/content/browser/renderer_host/backing_store_skia.cc
index 1c04363b70aabbdfd5592b99ac4850240a1d23a6..1302b6d048295020d773cd29a17f6c558c593346 100644
--- a/content/browser/renderer_host/backing_store_skia.cc
+++ b/content/browser/renderer_host/backing_store_skia.cc
@@ -60,6 +60,9 @@ void BackingStoreSkia::PaintToBackingStore(
   if (!dib)
     return;
 
+  SkPaint copy_paint;
+  copy_paint.setXfermodeMode(SkXfermode::kSrc_Mode);
+
   SkBitmap sk_bitmap;
   sk_bitmap.setConfig(SkBitmap::kARGB_8888_Config, width, height);
   sk_bitmap.setPixels(dib->memory());
@@ -73,7 +76,7 @@ void BackingStoreSkia::PaintToBackingStore(
     SkRect dstrect = SkRect::MakeXYWH(
         SkIntToScalar(copy_rect.x()), SkIntToScalar(copy_rect.y()),
         SkIntToScalar(w), SkIntToScalar(h));
-    canvas_.get()->drawBitmapRect(sk_bitmap, &srcrect, dstrect);
+    canvas_.get()->drawBitmapRect(sk_bitmap, &srcrect, dstrect, &copy_paint);
   }
 }