Fix smooth scroll animation flake on janky pages

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 996 matching lines @@
   if (animation_host_) {
     bool was_active = is_active_;
     is_active_ = false;
     for (size_t i = 0; i < animations_.size(); ++i) {
       if (animations_[i]->run_state() != Animation::WAITING_FOR_DELETION) {
         is_active_ = true;
         break;
       }
     }
 
-    if (is_active_ && (!was_active || force))
+    if (is_active_ && (!was_active || force)) {
       animation_host_->DidActivateElementAnimations(this);
-    else if (!is_active_ && (was_active || force))
+    } else if (!is_active_ && (was_active || force)) {
+      // Resetting last_tick_time_ here ensures that calling ::UpdateState
+      // before ::Animate doesn't start an animation.
+      last_tick_time_ = base::TimeTicks();
       animation_host_->DidDeactivateElementAnimations(this);
+    }
   }
 }
 
 void ElementAnimations::NotifyClientOpacityAnimated(
     float opacity,
     bool notify_active_elements,
     bool notify_pending_elements) {
   if (notify_active_elements && has_element_in_active_list())
     OnOpacityAnimated(ElementListType::ACTIVE, opacity);
   if (notify_pending_elements && has_element_in_pending_list())
@@ skipping 446 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