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

We have a situation with DeprecatedPaintLayerCompositor where it will never be in "compositingMode" (for a page with SVGImageChromeClient/EmptyClient)

BUG=394777
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1322853003

Cr-Commit-Position: refs/heads/master@{#347877}

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);
 }