CC Animation: Use ElementId to attach CC animation players.

cc/animation/element_animations.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/animation/element_animations.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 
@@ skipping 549 matching lines @@
              ->curve()
              ->ToScrollOffsetAnimationCurve()
              ->HasSetInitialValue()) {
       gfx::ScrollOffset current_scroll_offset;
       if (element_animations_impl->has_element_in_active_list()) {
         current_scroll_offset =
             element_animations_impl->ScrollOffsetForAnimation();
       } else {
         // The owning layer isn't yet in the active tree, so the main thread
         // scroll offset will be up-to-date.
+        DCHECK(has_element_in_active_list());
         current_scroll_offset = ScrollOffsetForAnimation();
       }
       animations_[i]->curve()->ToScrollOffsetAnimationCurve()->SetInitialValue(
           current_scroll_offset);
     }
 
     // The new animation should be set to run as soon as possible.
     Animation::RunState initial_run_state =
         Animation::WAITING_FOR_TARGET_AVAILABILITY;
     std::unique_ptr<Animation> to_add(
@@ skipping 771 matching lines @@
   if (animation_host()) {
     DCHECK(animation_host()->mutator_host_client());
     return animation_host()->mutator_host_client()->GetScrollOffsetForAnimation(
         element_id());
   }
 
   return gfx::ScrollOffset();
 }
 
 }  // namespace cc