Don't create render passes for transparent images.

cc/layer_impl.cc

 // 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.
 
 #include "cc/layer_impl.h"
 
 #include "base/debug/trace_event.h"
 #include "base/stringprintf.h"
 #include "cc/debug_border_draw_quad.h"
 #include "cc/debug_colors.h"
@@ skipping 96 matching lines @@
     m_children.clear();
 }
 
 void LayerImpl::createRenderSurface()
 {
     DCHECK(!m_renderSurface);
     m_renderSurface = make_scoped_ptr(new RenderSurfaceImpl(this));
     setRenderTarget(this);
 }
 
-bool LayerImpl::descendantDrawsContent()
-{
-    for (size_t i = 0; i < m_children.size(); ++i) {
-        if (m_children[i]->drawsContent() || m_children[i]->descendantDrawsContent())
-            return true;
-    }
-    return false;
-}
-
 scoped_ptr<SharedQuadState> LayerImpl::createSharedQuadState() const
 {
   scoped_ptr<SharedQuadState> state = SharedQuadState::Create();
   state->SetAll(m_drawTransform, m_visibleContentRect, m_drawableContentRect, m_clipRect, m_isClipped, m_drawOpacity);
   return state.Pass();
 }
 
 void LayerImpl::willDraw(ResourceProvider*)
 {
 #ifndef NDEBUG
@@ skipping 588 matching lines @@
 
 void LayerImpl::setVerticalScrollbarLayer(ScrollbarLayerImpl* scrollbarLayer)
 {
     if (!m_scrollbarAnimationController)
         m_scrollbarAnimationController = ScrollbarAnimationController::create(this);
     m_scrollbarAnimationController->setVerticalScrollbarLayer(scrollbarLayer);
     m_scrollbarAnimationController->updateScrollOffset(this);
 }
 
 }  // namespace cc