Use an auxiliary list of animation controllers.

cc/layer_tree_host_impl.h

Index: cc/layer_tree_host_impl.h
diff --git a/cc/layer_tree_host_impl.h b/cc/layer_tree_host_impl.h
index c05c30d2ed60cc44e2dd758038a48b73609383ac..05732a96c9e91b7184cb146168f77085f8e3249e 100644
--- a/cc/layer_tree_host_impl.h
+++ b/cc/layer_tree_host_impl.h
@@ -9,6 +9,7 @@
 #include "base/memory/scoped_ptr.h"
 #include "base/time.h"
 #include "cc/animation_events.h"
+#include "cc/animation_registrar.h"
 #include "cc/cc_export.h"
 #include "cc/input_handler.h"
 #include "cc/layer_sorter.h"
@@ -113,8 +114,10 @@ private:
 class CC_EXPORT LayerTreeHostImpl : public InputHandlerClient,
                                     public RendererClient,
                                     public TileManagerClient,
+                                    public AnimationRegistrar,
                                     public NON_EXPORTED_BASE(WebKit::WebCompositorOutputSurfaceClient) {
     typedef std::vector<LayerImpl*> LayerList;
+    typedef base::hash_set<LayerAnimationController*> AnimationControllerSet;
 
 public:
     static scoped_ptr<LayerTreeHostImpl> create(const LayerTreeSettings&, LayerTreeHostImplClient*, Proxy*);
@@ -240,9 +243,7 @@ public:
 
     bool hasTransparentBackground() const { return m_hasTransparentBackground; }
     void setHasTransparentBackground(bool transparent) { m_hasTransparentBackground = transparent; }
-
-    bool needsAnimateLayers() const { return m_needsAnimateLayers; }
-    void setNeedsAnimateLayers() { m_needsAnimateLayers = true; }
+    bool needsAnimateLayers() const { return !m_activeAnimationControllers.empty(); }
 
     void setNeedsRedraw();
 
@@ -330,6 +331,10 @@ private:
 
     void dumpRenderSurfaces(std::string*, int indent, const LayerImpl*) const;
 
+    // AnimationRegistar implementation.
+    virtual void Register(LayerAnimationController*) OVERRIDE;
+    virtual void Unregister(LayerAnimationController*) OVERRIDE;
+
     scoped_ptr<GraphicsContext> m_context;
     scoped_ptr<ResourceProvider> m_resourceProvider;
     scoped_ptr<Renderer> m_renderer;
@@ -352,7 +357,6 @@ private:
     bool m_hasTransparentBackground;
 
     // If this is true, it is necessary to traverse the layer tree ticking the animators.
-    bool m_needsAnimateLayers;
     bool m_pinchGestureActive;
     gfx::Point m_previousPinchAnchor;
 
@@ -377,6 +381,8 @@ private:
 
     size_t m_cumulativeNumLayersInLayerTree;
 
+    AnimationControllerSet m_activeAnimationControllers;
+
     DISALLOW_COPY_AND_ASSIGN(LayerTreeHostImpl);
 };