cc: Don't clear transparent piles to an opaque color

cc/layers/layer_impl.h

Index: cc/layers/layer_impl.h
diff --git a/cc/layers/layer_impl.h b/cc/layers/layer_impl.h
index e27e74352b403c3729afe8c34fa4d2fb37c546b7..7555e88f4f037631711ac8b251c08d80f18dffa1 100644
--- a/cc/layers/layer_impl.h
+++ b/cc/layers/layer_impl.h
@@ -144,6 +144,9 @@ class CC_EXPORT LayerImpl : LayerAnimationValueObserver {
 
   void SetBackgroundColor(SkColor background_color);
   SkColor background_color() const { return background_color_; }
+  // If contents_opaque(), return an opaque color else return a
+  // non-opaque color.  Tries to return background_color(), if possible.
+  SkColor SafeOpaqueBackgroundColor() const;
 
   void SetFilters(const WebKit::WebFilterOperations& filters);
   const WebKit::WebFilterOperations& filters() const { return filters_; }