CC Animations: Allow timelines and players without host to support non-compositingMode

cc/animation/animation_timeline.cc

Index: cc/animation/animation_timeline.cc
diff --git a/cc/animation/animation_timeline.cc b/cc/animation/animation_timeline.cc
index 9d19f8fbe0be53284056059c0023f473cc24e16d..c3f5e6918b7f502402d44423e905b4637f0d76b8 100644
--- a/cc/animation/animation_timeline.cc
+++ b/cc/animation/animation_timeline.cc
@@ -35,8 +35,8 @@ void AnimationTimeline::SetAnimationHost(AnimationHost* animation_host) {
 }
 
 void AnimationTimeline::AttachPlayer(scoped_refptr<AnimationPlayer> player) {
-  DCHECK(animation_host_);
   player->SetAnimationHost(animation_host_);
+
   player->SetAnimationTimeline(this);
   players_.push_back(player);
 }