Don't create render passes for transparent images.

cc/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_LAYER_IMPL_H_
 #define CC_LAYER_IMPL_H_
 
 #include <public/WebFilterOperations.h>
 #include <string>
 
@@ skipping 20 matching lines @@
 class QuadSink;
 class Renderer;
 class ScrollbarAnimationController;
 class ScrollbarLayerImpl;
 class Layer;
 
 struct AppendQuadsData;
 
 class CC_EXPORT LayerImpl : public LayerAnimationControllerClient {
 public:
+    typedef ScopedPtrVector<LayerImpl> LayerList;
+
     static scoped_ptr<LayerImpl> create(int id)
     {
         return make_scoped_ptr(new LayerImpl(id));
     }
 
     virtual ~LayerImpl();
 
     // LayerAnimationControllerClient implementation.
     virtual int id() const OVERRIDE;
     virtual void setOpacityFromAnimation(float) OVERRIDE;
     virtual float opacity() const OVERRIDE;
     virtual void setTransformFromAnimation(const gfx::Transform&) OVERRIDE;
     virtual const gfx::Transform& transform() const OVERRIDE;
 
     // Tree structure.
     LayerImpl* parent() { return m_parent; }
     const LayerImpl* parent() const { return m_parent; }
-    const ScopedPtrVector<LayerImpl>& children() const { return m_children; }
+    const LayerList& children() const { return m_children; }
     void addChild(scoped_ptr<LayerImpl>);
     void removeFromParent();
     void removeAllChildren();
 
     void setMaskLayer(scoped_ptr<LayerImpl>);
     LayerImpl* maskLayer() { return m_maskLayer.get(); }
     const LayerImpl* maskLayer() const { return m_maskLayer.get(); }
 
     void setReplicaLayer(scoped_ptr<LayerImpl>);
     LayerImpl* replicaLayer() { return m_replicaLayer.get(); }
@@ skipping 21 matching lines @@
     virtual RenderPass::Id firstContributingRenderPassId() const;
     virtual RenderPass::Id nextContributingRenderPassId(RenderPass::Id) const;
 
     // Returns true if this layer has content to draw.
     void setDrawsContent(bool);
     bool drawsContent() const { return m_drawsContent; }
 
     bool forceRenderSurface() const { return m_forceRenderSurface; }
     void setForceRenderSurface(bool force) { m_forceRenderSurface = force; }
 
-    // Returns true if any of the layer's descendants has content to draw.
-    virtual bool descendantDrawsContent();
+    // Returns 0 if none of the layer's descendants has content to draw,
+    // 1 if exactly one descendant has content to draw, or a number >1
+    // (but necessary the exact number of descendants) otherwise.
+    virtual int descendantsDrawContent();
 
     void setAnchorPoint(const gfx::PointF&);
     const gfx::PointF& anchorPoint() const { return m_anchorPoint; }
 
     void setAnchorPointZ(float);
     float anchorPointZ() const { return m_anchorPointZ; }
 
     void setBackgroundColor(SkColor);
     SkColor backgroundColor() const { return m_backgroundColor; }
 
@@ skipping 195 matching lines @@
 
     // Note carefully this does not affect the current layer.
     void noteLayerPropertyChangedForDescendants();
 
     virtual const char* layerTypeAsString() const;
 
     void dumpLayer(std::string*, int indent) const;
 
     // Properties internal to LayerImpl
     LayerImpl* m_parent;
-    ScopedPtrVector<LayerImpl> m_children;
+    LayerList m_children;
     // m_maskLayer can be temporarily stolen during tree sync, we need this ID to confirm newly assigned layer is still the previous one
     int m_maskLayerId;
     scoped_ptr<LayerImpl> m_maskLayer;
     int m_replicaLayerId; // ditto
     scoped_ptr<LayerImpl> m_replicaLayer;
     int m_layerId;
     LayerTreeHostImpl* m_layerTreeHostImpl;
 
     // Properties synchronized from the associated Layer.
     gfx::PointF m_anchorPoint;
@@ skipping 98 matching lines @@
     scoped_ptr<ScrollbarAnimationController> m_scrollbarAnimationController;
 
     DISALLOW_COPY_AND_ASSIGN(LayerImpl);
 };
 
 void sortLayers(std::vector<LayerImpl*>::iterator first, std::vector<LayerImpl*>::iterator end, LayerSorter*);
 
 }
 
 #endif  // CC_LAYER_IMPL_H_