Add guard pages in front of platform allocations

src/platform.h

Index: src/platform.h
===================================================================
--- src/platform.h	(revision 8619)
+++ src/platform.h	(working copy)
@@ -203,6 +203,7 @@
                         size_t* allocated,
                         bool is_executable);
   static void Free(void* address, const size_t size);
+
   // Get the Alignment guaranteed by Allocate().
   static size_t AllocateAlignment();
 
@@ -210,6 +211,9 @@
   // Protect/unprotect a block of memory by marking it read-only/writable.
   static void Protect(void* address, size_t size);
   static void Unprotect(void* address, size_t size, bool is_executable);
+
+  // Assign memory as a guard page so that access will cause an exception.
+  static void Guard(void* address, const size_t size);
 #endif
 
   // Returns an indication of whether a pointer is in a space that