Ensure that executable pages are properly guarded.

src/platform-linux.cc

Index: src/platform-linux.cc
diff --git a/src/platform-linux.cc b/src/platform-linux.cc
index 14297483c3ad401a48ae6d8417f70c8941f889b1..0da1c08fdd5cb0f63246e69e6b034acdb10348ea 100644
--- a/src/platform-linux.cc
+++ b/src/platform-linux.cc
@@ -666,6 +666,12 @@ bool VirtualMemory::Uncommit(void* address, size_t size) {
 }
 
 
+bool VirtualMemory::Guard(void* address) {
+  OS::Guard(address, OS::CommitPageSize());
+  return true;
+}
+
+
 void* VirtualMemory::ReserveRegion(size_t size) {
   void* result = mmap(OS::GetRandomMmapAddr(),
                       size,