cc: Drop ContextProvider references on the impl thread before quitting

cc/trees/layer_tree_host_impl.h

Index: cc/trees/layer_tree_host_impl.h
diff --git a/cc/trees/layer_tree_host_impl.h b/cc/trees/layer_tree_host_impl.h
index 488cc3688692a4fb8cdc4e3c398e6149422bc7b6..fc852a37ed8d17395c9eaf8712b51977279c8c66 100644
--- a/cc/trees/layer_tree_host_impl.h
+++ b/cc/trees/layer_tree_host_impl.h
@@ -46,6 +46,7 @@ class PaintTimeCounter;
 class MemoryHistory;
 class RenderingStatsInstrumentation;
 class RenderPassDrawQuad;
+class TextureMailboxDeleter;
 class TopControlsManager;
 class UIResourceBitmap;
 struct RendererCapabilities;
@@ -538,6 +539,8 @@ class CC_EXPORT LayerTreeHostImpl
   scoped_ptr<MemoryHistory> memory_history_;
   scoped_ptr<DebugRectHistory> debug_rect_history_;
 
+  scoped_ptr<TextureMailboxDeleter> texture_mailbox_deleter_;
+
   // The maximum memory that would be used by the prioritized resource
   // manager, if there were no limit on memory usage.
   size_t max_memory_needed_bytes_;