cc: Add some more infrastructure for two trees

PictureLayerImpl gets the ability to sync a tiling from active
to pending tree when it gets created so that layers can share tiles.
A few functions are added to make this possible.

BUG=155209


Review URL: https://chromiumcodereview.appspot.com/11574026

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@173366 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/cc/layer_tree_host_impl.h b/cc/layer_tree_host_impl.h
index 9943da2..79c26c02 100644
--- a/cc/layer_tree_host_impl.h
+++ b/cc/layer_tree_host_impl.h
@@ -204,6 +204,7 @@
     void readback(void* pixels, const gfx::Rect&);
 
     LayerTreeImpl* activeTree() { return m_activeTree.get(); }
+    LayerTreeImpl* pendingTree() { return m_pendingTree.get(); }
 
     // TODO(nduca): Remove these in favor of LayerTreeImpl.
     void setRootLayer(scoped_ptr<LayerImpl>);
@@ -354,6 +355,7 @@
     scoped_ptr<Renderer> m_renderer;
     scoped_ptr<TileManager> m_tileManager;
 
+    scoped_ptr<LayerTreeImpl> m_pendingTree;
     scoped_ptr<LayerTreeImpl> m_activeTree;
 
     bool m_scrollDeltaIsInViewportSpace;