Make --disable-gpu-vsync work with CoreAnimation

content/browser/renderer_host/render_widget_host_view_mac.mm

 // Copyright (c) 2012 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/renderer_host/render_widget_host_view_mac.h"
 
 #import <objc/runtime.h>
 #include <QuartzCore/QuartzCore.h>
 
 #include "base/basictypes.h"
@@ skipping 1873 matching lines @@
   weak_factory_.InvalidateWeakPtrs();
   render_widget_host_->Shutdown();
   // Do not touch any members at this point, |this| has been deleted.
 }
 
 void RenderWidgetHostViewMac::GotAcceleratedFrame() {
   // Update the host with VSync parametrs.
   base::TimeTicks timebase;
   base::TimeDelta interval;
   if (compositing_iosurface_context_ &&
+      compositing_iosurface_context_->display_link() &&
       compositing_iosurface_context_->display_link()->GetVSyncParameters(
           &timebase, &interval)) {
     render_widget_host_->UpdateVSyncParameters(timebase, interval);
   }
 
   if (!last_frame_was_accelerated_) {
     last_frame_was_accelerated_ = true;
 
     if (!use_core_animation_) {
       // Need to wipe the software view with transparency to expose the GL
@@ skipping 2231 matching lines @@
   return YES;
 }
 
 - (BOOL)isOpaque {
   if (renderWidgetHostView_->use_core_animation_)
     return YES;
   return [super isOpaque];
 }
 
 @end