Landing: Write protect executable pages in the VM.

runtime/vm/virtual_memory.h

Index: runtime/vm/virtual_memory.h
===================================================================
--- runtime/vm/virtual_memory.h	(revision 31654)
+++ runtime/vm/virtual_memory.h	(working copy)
@@ -41,7 +41,10 @@
   }
 
   // Changes the protection of the virtual memory area.
-  bool Protect(Protection mode);
+  static bool Protect(void* address, intptr_t size, Protection mode);
+  bool Protect(Protection mode) {
+    return Protect(address(), size(), mode);
+  }
 
   // Reserves a virtual memory segment with size. If a segment of the requested
   // size cannot be allocated NULL is returned.
@@ -57,6 +60,8 @@
     return page_size_;
   }
 
+  static bool InSamePage(uword address0, uword address1);
+
  private:
   // Truncate this virtual memory segment.
   void Truncate(uword new_start, intptr_t size);