cc: Implement shared worker contexts. (v1)

cc/trees/layer_tree_host_impl.cc

 // Copyright 2011 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/trees/layer_tree_host_impl.h"
 
 #include <algorithm>
 #include <limits>
 #include <map>
 #include <set>
@@ skipping 1595 matching lines @@
   return settings_.gpu_rasterization_msaa_sample_count;
 }
 
 bool LayerTreeHostImpl::CanUseGpuRasterization() {
   if (!(output_surface_ && output_surface_->context_provider() &&
         output_surface_->worker_context_provider()))
     return false;
 
   ContextProvider* context_provider =
       output_surface_->worker_context_provider();
-  base::AutoLock context_lock(*context_provider->GetLock());
+  ContextProvider::ScopedContextLock scoped_context(context_provider);
+  if (context_provider->HasBeenDestroyed())
+    return false;
   if (!context_provider->GrContext())
     return false;
 
   return true;
 }
 
 void LayerTreeHostImpl::UpdateGpuRasterizationStatus() {
   bool use_gpu = false;
   bool use_msaa = false;
   bool using_msaa_for_complex_content =
@@ skipping 1971 matching lines @@
   if (active_tree()) {
     LayerAnimationValueProvider* layer = active_tree()->LayerById(layer_id);
     if (layer)
       return layer->ScrollOffsetForAnimation();
   }
 
   return gfx::ScrollOffset();
 }
 
 }  // namespace cc