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

cc/layers/picture_layer.cc

 // Copyright 2012 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 "cc/layers/picture_layer.h"
 
 #include "cc/debug/devtools_instrumentation.h"
 #include "cc/layers/picture_layer_impl.h"
 #include "cc/trees/layer_tree_impl.h"
 #include "ui/gfx/rect_conversions.h"
@@ skipping 74 matching lines @@
   // Calling paint in WebKit can sometimes cause invalidations, so save
   // off the invalidation prior to calling update.
   pile_invalidation_.Swap(&pending_invalidation_);
   pending_invalidation_.Clear();
 
   gfx::Rect visible_layer_rect = gfx::ScaleToEnclosingRect(
       visible_content_rect(), 1.f / contents_scale_x());
   devtools_instrumentation::ScopedLayerTask paint_layer(
       devtools_instrumentation::kPaintLayer, id());
   pile_->Update(client_,
-                background_color(),
+                SafeOpaqueBackgroundColor(),
                 pile_invalidation_,
                 visible_layer_rect,
                 stats);
 }
 
 void PictureLayer::SetIsMask(bool is_mask) {
   is_mask_ = is_mask;
 }
 
 bool PictureLayer::SupportsLCDText() const {
   return true;
 }
 
 }  // namespace cc