Add a guard page in front of metadata allocations.

third_party/tcmalloc/chromium/src/system-alloc.h

 // Copyright (c) 2005, Google Inc.
 // All rights reserved.
 // 
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 // 
 //     * Redistributions of source code must retain the above copyright
 // notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
@@ skipping 59 matching lines @@
 // performance.  (Only pages fully covered by the memory region will
 // be released, partial pages will not.)
 extern void TCMalloc_SystemRelease(void* start, size_t length);
 
 // Called to ressurect memory which has been previously released
 // to the system via TCMalloc_SystemRelease.  An attempt to
 // commit a page that is already committed does not cause this
 // function to fail.
 extern void TCMalloc_SystemCommit(void* start, size_t length);
 
+// Guards the first page in the supplied range of memory and returns the size
+// of the guard page. Will return 0 if a guard cannot be added to the page
+// (e.g. start is not aligned or size is not large enough).
+extern size_t TCMalloc_SystemAddGuard(void* start, size_t size);
+
 // The current system allocator.
 extern PERFTOOLS_DLL_DECL SysAllocator* sys_alloc;
 
 #endif /* TCMALLOC_SYSTEM_ALLOC_H_ */