Ensure that executable pages are properly guarded.

src/platform.h

Index: src/platform.h
diff --git a/src/platform.h b/src/platform.h
index a0186d580fe32c6573d7dc3031011bd0244b0ea8..38e633a38081e155744d9e1942c9eee8fc73827f 100644
--- a/src/platform.h
+++ b/src/platform.h
@@ -356,6 +356,9 @@ class VirtualMemory {
   // Uncommit real memory.  Returns whether the operation succeeded.
   bool Uncommit(void* address, size_t size);
 
+  // Creates a single guard page at the given address.
+  bool Guard(void* address);
+
   void Release() {
     ASSERT(IsReserved());
     // Notice: Order is important here. The VirtualMemory object might live