Fix OverscrollWindowDelegate to not dispatch superfluous OverscrollModeChange events

content/browser/web_contents/aura/overscroll_window_animation.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 "content/browser/web_contents/aura/overscroll_window_animation.h"
 
 #include <algorithm>
 
 #include "base/i18n/rtl.h"
 #include "content/browser/web_contents/aura/shadow_layer_delegate.h"
@@ skipping 68 matching lines @@
     overscroll_cancelled_ = false;
   } else {
     delegate_->OnOverscrollCompleted(slide_window_.Pass());
   }
   direction_ = SLIDE_NONE;
 }
 
 void OverscrollWindowAnimation::OnOverscrollModeChange(
     OverscrollMode old_mode,
     OverscrollMode new_mode) {
+  DCHECK_NE(old_mode, new_mode);
   Direction new_direction = GetDirectionForMode(new_mode);
   if (new_direction == SLIDE_NONE) {
     // The user cancelled the in progress animation.
     if (is_active())
       CancelSlide();
     return;
   }
   if (is_active()) {
     slide_window_->layer()->GetAnimator()->StopAnimating();
     delegate_->GetMainWindow()->layer()->GetAnimator()->StopAnimating();
@@ skipping 68 matching lines @@
 ui::Layer* OverscrollWindowAnimation::GetBackLayer() const {
   DCHECK(direction_ != SLIDE_NONE);
   if (direction_ == SLIDE_BACK) {
     DCHECK(slide_window_);
     return slide_window_->layer();
   }
   return delegate_->GetMainWindow()->layer();
 }
 
 }  // namespace content