CC Animation: Start replacing int layer_id with ElementId element_id.

cc/test/animation_timelines_test_common.cc

Index: cc/test/animation_timelines_test_common.cc
diff --git a/cc/test/animation_timelines_test_common.cc b/cc/test/animation_timelines_test_common.cc
index 70f608286ae2411d1f5d28394392c7906ecb02c4..f0ad7ae958f75c961c6863147d21320d8915e5db 100644
--- a/cc/test/animation_timelines_test_common.cc
+++ b/cc/test/animation_timelines_test_common.cc
@@ -295,7 +295,7 @@ AnimationTimelinesTest::AnimationTimelinesTest()
   host_ = client_.host();
   host_impl_ = client_impl_.host();
 
-  layer_id_ = NextTestLayerId();
+  element_id_ = NextTestLayerId();
 }
 
 AnimationTimelinesTest::~AnimationTimelinesTest() {
@@ -323,18 +323,18 @@ void AnimationTimelinesTest::CreateTestLayer(
 }
 
 void AnimationTimelinesTest::CreateTestMainLayer() {
-  client_.RegisterLayer(layer_id_, LayerTreeType::ACTIVE);
+  client_.RegisterLayer(element_id_, LayerTreeType::ACTIVE);
 }
 
 void AnimationTimelinesTest::CreateTestImplLayer(
     LayerTreeType layer_tree_type) {
-  client_impl_.RegisterLayer(layer_id_, layer_tree_type);
+  client_impl_.RegisterLayer(element_id_, layer_tree_type);
 }
 
 void AnimationTimelinesTest::AttachTimelinePlayerLayer() {
   host_->AddAnimationTimeline(timeline_);
   timeline_->AttachPlayer(player_);
-  player_->AttachLayer(layer_id_);
+  player_->AttachLayer(element_id_);
 }
 
 void AnimationTimelinesTest::CreateImplTimelineAndPlayer() {