cc: Add SwapPromiseManager and SurfaceSequenceGenerator.

cc/trees/layer_tree.h

Index: cc/trees/layer_tree.h
diff --git a/cc/trees/layer_tree.h b/cc/trees/layer_tree.h
index e731d4b1c9d900dddb3c53b791a39a2b4652b629..06e95458e8a63b7ddb0a8387a39d179196e3d3f2 100644
--- a/cc/trees/layer_tree.h
+++ b/cc/trees/layer_tree.h
@@ -38,6 +38,8 @@ class Layer;
 class LayerTreeHost;
 class LayerTreeImpl;
 struct PendingPageScaleAnimation;
+class SwapPromiseManager;
+class SurfaceSequenceGenerator;
 
 class CC_EXPORT LayerTree : public MutatorHostClient {
  public:
@@ -120,6 +122,9 @@ class CC_EXPORT LayerTree : public MutatorHostClient {
 
   void SetNeedsDisplayOnAllLayers();
 
+  SwapPromiseManager* GetSwapPromiseManager() const;
+  SurfaceSequenceGenerator* GetSurfaceSequenceGenerator() const;
+
   // Methods which should only be used internally in cc ------------------
   void RegisterLayer(Layer* layer);
   void UnregisterLayer(Layer* layer);