Avoid destroying surface before the parent surface stops referencing it.

cc/surfaces/surface_id_allocator.h

 // Copyright 2014 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.
 
 #ifndef CC_SURFACES_SURFACE_ID_ALLOCATOR_H_
 #define CC_SURFACES_SURFACE_ID_ALLOCATOR_H_
 
 #include "cc/surfaces/surface_id.h"
 #include "cc/surfaces/surfaces_export.h"
 
 namespace cc {
 
 // This is a helper class for generating surface IDs within a specified
 // namespace.  This is not threadsafe, to use from multiple threads wrap this
 // class in a mutex.
 class CC_SURFACES_EXPORT SurfaceIdAllocator {
  public:
   explicit SurfaceIdAllocator(uint32_t id_namespace);
 
   SurfaceId GenerateId();
 
   static uint32_t NamespaceForId(SurfaceId id);
 
+  uint32_t id_namespace() const { return id_namespace_; }
+
  private:
   uint32_t id_namespace_;
   uint32_t next_id_;
 
   DISALLOW_COPY_AND_ASSIGN(SurfaceIdAllocator);
 };
 
 }  // namespace cc
 
 #endif  // CC_SURFACES_SURFACE_ID_ALLOCATOR_H_