cc : Fix the bug in tracking currently running animations for an element

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 617 matching lines @@
   ElementAnimations::PlayersList::Iterator it(players_list_.get());
   AnimationPlayer* player;
   while ((player = it.GetNext()) != nullptr) {
     for (const auto& animation : player->animations()) {
       if (!animation->is_finished() &&
           animation->target_property() == property) {
         animation_state->potentially_animating_for_active_elements |=
             animation->affects_active_elements();
         animation_state->potentially_animating_for_pending_elements |=
             animation->affects_pending_elements();
-        animation_state->currently_running_for_active_elements =
-            animation_state->potentially_animating_for_active_elements &&
+        animation_state->currently_running_for_active_elements |=
+            animation->affects_active_elements() &&
             animation->InEffect(last_tick_time_);
-        animation_state->currently_running_for_pending_elements =
-            animation_state->potentially_animating_for_pending_elements &&
+        animation_state->currently_running_for_pending_elements |=
+            animation->affects_pending_elements() &&
             animation->InEffect(last_tick_time_);

[loyso (OOO), 2016/09/14 23:49:08]

NIT: Would be nice to add a DCHECK as a loop invariant.
Given that p = potentially_animating_for_pending_elements, c =
currently_running_for_active_elements, c must be a subset of p (c <= p).
i.e.:
// DNF: (!p && !c) || (p && !c) || (p && c)
DCHECK_LE(c, p);

In C++ you can compare the order of bool values, they are promoted to int as
false -> 0, true -> 1.

       }
     }
   }
 
   bool potentially_animating_changed_for_active_elements =
       was_potentially_animating_for_active_elements !=
       animation_state->potentially_animating_for_active_elements;
   bool potentially_animating_changed_for_pending_elements =
       was_potentially_animating_for_pending_elements !=
       animation_state->potentially_animating_for_pending_elements;
@@ skipping 209 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