CC Animation: Start replacing int layer_id with ElementId element_id.

cc/animation/animation_events.cc

Index: cc/animation/animation_events.cc
diff --git a/cc/animation/animation_events.cc b/cc/animation/animation_events.cc
index e747f8bec2a06745180b8dda0edc335e241d0434..8c39e26a776cc9f74a4f48160b7b769f176a9789 100644
--- a/cc/animation/animation_events.cc
+++ b/cc/animation/animation_events.cc
@@ -7,12 +7,12 @@
 namespace cc {
 
 AnimationEvent::AnimationEvent(AnimationEvent::Type type,
-                               int layer_id,
+                               ElementId element_id,
                                int group_id,
                                TargetProperty::Type target_property,
                                base::TimeTicks monotonic_time)
     : type(type),
-      layer_id(layer_id),
+      element_id(element_id),
       group_id(group_id),
       target_property(target_property),
       monotonic_time(monotonic_time),
@@ -21,7 +21,7 @@ AnimationEvent::AnimationEvent(AnimationEvent::Type type,
 
 AnimationEvent::AnimationEvent(const AnimationEvent& other) {
   type = other.type;
-  layer_id = other.layer_id;
+  element_id = other.element_id;
   group_id = other.group_id;
   target_property = other.target_property;
   monotonic_time = other.monotonic_time;
@@ -36,7 +36,7 @@ AnimationEvent::AnimationEvent(const AnimationEvent& other) {
 
 AnimationEvent& AnimationEvent::operator=(const AnimationEvent& other) {
   type = other.type;
-  layer_id = other.layer_id;
+  element_id = other.element_id;
   group_id = other.group_id;
   target_property = other.target_property;
   monotonic_time = other.monotonic_time;