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

cc/layers/layer_impl.h

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_LAYERS_LAYER_IMPL_H_
 #define CC_LAYERS_LAYER_IMPL_H_
 
 #include <string>
 
 #include "base/logging.h"
@@ skipping 126 matching lines @@
   void SetForceRenderSurface(bool force) { force_render_surface_ = force; }
 
   void SetAnchorPoint(gfx::PointF anchor_point);
   gfx::PointF anchor_point() const { return anchor_point_; }
 
   void SetAnchorPointZ(float anchor_point_z);
   float anchor_point_z() const { return anchor_point_z_; }
 
   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_; }
 
   void SetBackgroundFilters(const WebKit::WebFilterOperations& filters);
   const WebKit::WebFilterOperations& background_filters() const {
     return background_filters_;
   }
 
   void SetFilter(const skia::RefPtr<SkImageFilter>& filter);
@@ skipping 408 matching lines @@
   // Group of properties that need to be computed based on the layer tree
   // hierarchy before layers can be drawn.
   DrawProperties<LayerImpl, RenderSurfaceImpl> draw_properties_;
 
   DISALLOW_COPY_AND_ASSIGN(LayerImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_LAYER_IMPL_H_