CC Animations: Fix the case when main-thread player changes layer_id.

cc/test/animation_timelines_test_common.h

Index: cc/test/animation_timelines_test_common.h
diff --git a/cc/test/animation_timelines_test_common.h b/cc/test/animation_timelines_test_common.h
index 8b3996b3ccbe83d636cd9cb66eeea9c38c561537..518929d6abc8804f17b9f9c6901c29fdebba3695 100644
--- a/cc/test/animation_timelines_test_common.h
+++ b/cc/test/animation_timelines_test_common.h
@@ -174,6 +174,8 @@ class AnimationTimelinesTest : public testing::Test {
   AnimationPlayer* GetPlayerForLayerId(int layer_id);
   AnimationPlayer* GetImplPlayerForLayerId(int layer_id);
 
+  int NextTestLayerId();
+
   TestHostClient client_;
   TestHostClient client_impl_;
 
@@ -182,7 +184,9 @@ class AnimationTimelinesTest : public testing::Test {
 
   const int timeline_id_;
   const int player_id_;
-  const int layer_id_;
+  int layer_id_;
+
+  int next_test_layer_id_;
 
   scoped_refptr<AnimationTimeline> timeline_;
   scoped_refptr<AnimationPlayer> player_;