Activate LayerImpl tree with sync+push instead of pointer swap

cc/layer_tree_host_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_TREE_HOST_IMPL_H_
 #define CC_LAYER_TREE_HOST_IMPL_H_
 
 #include "base/basictypes.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/time.h"
@@ skipping 151 matching lines @@
     const RendererCapabilities& rendererCapabilities() const;
 
     bool swapBuffers();
 
     void readback(void* pixels, const gfx::Rect&);
 
     LayerTreeImpl* activeTree() { return m_activeTree.get(); }
     const LayerTreeImpl* activeTree() const { return m_activeTree.get(); }
     LayerTreeImpl* pendingTree() { return m_pendingTree.get(); }
     const LayerTreeImpl* pendingTree() const { return m_pendingTree.get(); }
+    const LayerTreeImpl* recycleTree() const { return m_recycleTree.get(); }
     void createPendingTree();
     void checkForCompletedTileUploads();
     virtual void activatePendingTreeIfNeeded();
 
     // Shortcuts to layers on the active tree.
     LayerImpl* rootLayer() const;
     LayerImpl* rootScrollLayer() const;
     LayerImpl* currentlyScrollingLayer() const;
 
     bool visible() const { return m_visible; }
@@ skipping 126 matching lines @@
 
     void dumpRenderSurfaces(std::string*, int indent, const LayerImpl*) const;
 
     static LayerImpl* getNonCompositedContentLayerRecursive(LayerImpl* layer);
 
     scoped_ptr<OutputSurface> m_outputSurface;
     scoped_ptr<ResourceProvider> m_resourceProvider;
     scoped_ptr<Renderer> m_renderer;
     scoped_ptr<TileManager> m_tileManager;
 
+    // Tree currently being drawn.
+    scoped_ptr<LayerTreeImpl> m_activeTree;
+
+    // In impl-side painting mode, tree with possibly incomplete rasterized
+    // content. May be promoted to active by activatePendingTreeIfNeeded().
     scoped_ptr<LayerTreeImpl> m_pendingTree;
-    scoped_ptr<LayerTreeImpl> m_activeTree;
+
+    // In impl-side painting mode, inert tree with layers that can be recycled
+    // by the next sync from the main thread.
+    scoped_ptr<LayerTreeImpl> m_recycleTree;
 
     bool m_scrollDeltaIsInViewportSpace;
     LayerTreeSettings m_settings;
     LayerTreeDebugState m_debugState;
     gfx::Size m_layoutViewportSize;
     gfx::Size m_deviceViewportSize;
     float m_deviceScaleFactor;
     bool m_visible;
     ManagedMemoryPolicy m_managedMemoryPolicy;
 
@@ skipping 26 matching lines @@
     size_t m_lastSentMemoryUseBytes;
 
     scoped_ptr<AnimationRegistrar> m_animationRegistrar;
 
     DISALLOW_COPY_AND_ASSIGN(LayerTreeHostImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYER_TREE_HOST_IMPL_H_