Landing: Write protect executable pages in the VM.

runtime/vm/assembler.h

Index: runtime/vm/assembler.h
===================================================================
--- runtime/vm/assembler.h	(revision 32487)
+++ runtime/vm/assembler.h	(working copy)
@@ -53,6 +53,8 @@
  public:
   virtual void Process(const MemoryRegion& region, intptr_t position) = 0;
 
+  virtual bool IsPointerOffset() const = 0;
+
   // It would be ideal if the destructor method could be made private,
   // but the g++ compiler complains when this is subclassed.
   virtual ~AssemblerFixup() { UNREACHABLE(); }
@@ -118,6 +120,10 @@
     fixup_ = fixup;
   }
 
+  // Count the fixups that produce a pointer offset, without processing
+  // the fixups.
+  intptr_t CountPointerOffsets() const;
+
   // Get the size of the emitted code.
   intptr_t Size() const { return cursor_ - contents_; }
   uword contents() const { return contents_; }