Include stddef.h for the definition of size_t.

base/allocator/allocator_shim.h

 // 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.
 
 #ifndef BASE_ALLOCATOR_ALLOCATOR_SHIM_H_
 #define BASE_ALLOCATOR_ALLOCATOR_SHIM_H_
 
+#include <stddef.h>
+
 namespace base {
 namespace allocator {
 
 // Resets the environment variable CHROME_ALLOCATOR to specify the choice to
 // be used by subprocesses.  Priority is given to the current value of
 // CHROME_ALLOCATOR_2 (if specified), then CHROME_ALLOCATOR (if specified), and
 // then a default value (typically set to TCMALLOC).
 void SetupSubprocessAllocator();
 
 // Expose some of tcmalloc functions for test.
 void* TCMallocDoMallocForTest(size_t size);
 void TCMallocDoFreeForTest(void* ptr);
 size_t ExcludeSpaceForMarkForTest(size_t size);
 
 }  // namespace allocator.
 }  // namespace base.
 
 #endif   // BASE_ALLOCATOR_ALLOCATOR_SHIM_H_