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

cc/trees/layer_tree_host.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_TREES_LAYER_TREE_HOST_H_
 #define CC_TREES_LAYER_TREE_HOST_H_
 
 #include <limits>
 #include <vector>
 
@@ skipping 177 matching lines @@
   void SetViewportSize(gfx::Size device_viewport_size);
   void SetOverdrawBottomHeight(float overdraw_bottom_height);
 
   gfx::Size device_viewport_size() const { return device_viewport_size_; }
   float overdraw_bottom_height() const { return overdraw_bottom_height_; }
 
   void SetPageScaleFactorAndLimits(float page_scale_factor,
                                    float min_page_scale_factor,
                                    float max_page_scale_factor);
 
+  SkColor background_color() const { return background_color_; }
   void set_background_color(SkColor color) { background_color_ = color; }
 
   void set_has_transparent_background(bool transparent) {
     has_transparent_background_ = transparent;
   }
 
   PrioritizedResourceManager* contents_texture_manager() const {
     return contents_texture_manager_.get();
   }
 
@@ skipping 155 matching lines @@
     int64 total_num_cc_layers_will_use_lcd_text;
   };
   LCDTextMetrics lcd_text_metrics_;
 
   DISALLOW_COPY_AND_ASSIGN(LayerTreeHost);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_HOST_H_