Add a guard page in front of metadata allocations.

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

Index: third_party/tcmalloc/chromium/src/system-alloc.h
===================================================================
--- third_party/tcmalloc/chromium/src/system-alloc.h	(revision 110522)
+++ third_party/tcmalloc/chromium/src/system-alloc.h	(working copy)
@@ -77,6 +77,11 @@
 // 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;