CC Animation: Start replacing int layer_id with ElementId element_id.

cc/animation/animation_events.h

Index: cc/animation/animation_events.h
diff --git a/cc/animation/animation_events.h b/cc/animation/animation_events.h
index f4728ba6f6b44b0a6caa32b1e06887cbb3668830..fc0c41e9d1cf921a5279fbba075f55d1e488bc58 100644
--- a/cc/animation/animation_events.h
+++ b/cc/animation/animation_events.h
@@ -12,6 +12,7 @@
 #include "cc/animation/animation_curve.h"
 #include "cc/base/cc_export.h"
 #include "cc/output/filter_operations.h"
+#include "cc/trees/mutator_host_client.h"
 #include "ui/gfx/transform.h"
 
 namespace cc {
@@ -20,7 +21,7 @@ struct CC_EXPORT AnimationEvent {
   enum Type { STARTED, FINISHED, ABORTED, PROPERTY_UPDATE, TAKEOVER };
 
   AnimationEvent(Type type,
-                 int layer_id,
+                 ElementId element_id,
                  int group_id,
                  TargetProperty::Type target_property,
                  base::TimeTicks monotonic_time);
@@ -31,7 +32,7 @@ struct CC_EXPORT AnimationEvent {
   ~AnimationEvent();
 
   Type type;
-  int layer_id;
+  ElementId element_id;
   int group_id;
   TargetProperty::Type target_property;
   base::TimeTicks monotonic_time;