Revert of Use element id's for animations

cc/animation/animation_host.h

Index: cc/animation/animation_host.h
diff --git a/cc/animation/animation_host.h b/cc/animation/animation_host.h
index 318baf0be273b6e20b969dad16bdbafa4331bde1..8f5ff14a725896cf659af1c1e2be403ab2d98cee 100644
--- a/cc/animation/animation_host.h
+++ b/cc/animation/animation_host.h
@@ -45,9 +45,7 @@
 class CC_EXPORT AnimationHost {
  public:
   using ElementToAnimationsMap =
-      std::unordered_map<ElementId,
-                         scoped_refptr<ElementAnimations>,
-                         ElementIdHash>;
+      std::unordered_map<ElementId, scoped_refptr<ElementAnimations>>;
 
   static std::unique_ptr<AnimationHost> Create(ThreadInstance thread_instance);
   ~AnimationHost();