Ensure that blending is disabled for layers with no alpha

ui/gfx/compositor/layer.cc

 // Copyright (c) 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.
 
 #include "ui/gfx/compositor/layer.h"
 
 #include <algorithm>
 
 #include "base/logging.h"
 
@@ skipping 71 matching lines @@
 
 void Layer::Draw() {
   ui::TextureDrawParams texture_draw_params;
   for (Layer* layer = this; layer; layer = layer->parent_) {
     texture_draw_params.transform.ConcatTransform(layer->transform_);
     texture_draw_params.transform.ConcatTranslate(
         static_cast<float>(layer->bounds_.x()),
         static_cast<float>(layer->bounds_.y()));
   }
 
-  // Only blend for child layers. The root layer will clobber the cleared bg.
-  texture_draw_params.blend = parent_ != NULL;
+  // Only blend for transparent child layers.
+  // The root layer will clobber the cleared bg.
+  texture_draw_params.blend = parent_ != NULL && !fills_bounds_opaquely_;
 
 #if defined(OS_WIN)
   texture_->Draw(texture_draw_params);
 #else
   hole_rect_ = hole_rect_.Intersect(
       gfx::Rect(0, 0, bounds_.width(), bounds_.height()));
 
   // top
   DrawRegion(texture_draw_params, gfx::Rect(0,
                                             0,
@@ skipping 32 matching lines @@
         !children_[i]->transform().HasChange()) {
       hole_rect_ = children_[i]->bounds();
       return;
     }
   }
   // no opaque child layers, set hole_rect_ to empty
   hole_rect_ = gfx::Rect();
 }
 
 }  // namespace ui