Landing: Write protect executable pages in the VM.

runtime/vm/assembler_x64.h

Index: runtime/vm/assembler_x64.h
===================================================================
--- runtime/vm/assembler_x64.h	(revision 32487)
+++ runtime/vm/assembler_x64.h	(working copy)
@@ -752,6 +752,13 @@
 
   intptr_t CodeSize() const { return buffer_.Size(); }
   intptr_t prologue_offset() const { return prologue_offset_; }
+
+  // Count the fixups that produce a pointer offset, without processing
+  // the fixups.
+  intptr_t CountPointerOffsets() const {
+    return buffer_.CountPointerOffsets();
+  }
+
   const ZoneGrowableArray<intptr_t>& GetPointerOffsets() const {
     return buffer_.pointer_offsets();
   }