Ensure that executable pages are properly guarded.

src/platform-win32.cc

Index: src/platform-win32.cc
diff --git a/src/platform-win32.cc b/src/platform-win32.cc
index 68c393884b060c7da68b4a8e8788d05d950bcdb5..fdd4eb082abd9cb29b4db326a2b453a463c018e0 100644
--- a/src/platform-win32.cc
+++ b/src/platform-win32.cc
@@ -1504,6 +1504,17 @@ bool VirtualMemory::CommitRegion(void* base, size_t size, bool is_executable) {
 }
 
 
+bool VirtualMemory::Guard(void* address) {
+  if (NULL == VirtualAlloc(address,
+                           OS::CommitPageSize(),
+                           MEM_COMMIT,
+                           PAGE_READONLY | PAGE_GUARD)) {
+    return false;
+  }
+  return true;
+}
+
+
 bool VirtualMemory::UncommitRegion(void* base, size_t size) {
   return VirtualFree(base, size, MEM_DECOMMIT) != 0;
 }