GPU: 'Pass' SharedMemory when possible.

gpu/command_buffer/common/buffer.cc

 // 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.
 
 #include "gpu/command_buffer/common/buffer.h"
 
+#include "base/logging.h"
+
 namespace gpu {
 
 Buffer::Buffer(scoped_ptr<base::SharedMemory> shared_memory, size_t size)
     : shared_memory_(shared_memory.Pass()),
       memory_(shared_memory_->memory()),
-      size_(size) {}
+      size_(size) {
+  DCHECK(memory_) << "The memory must be mapped to create a Buffer";
+}
 
 Buffer::~Buffer() {}
 
 } // namespace gpu