Add VerifiedMemory helper for write-barrier verification.

runtime/vm/virtual_memory.h

Index: runtime/vm/virtual_memory.h
===================================================================
--- runtime/vm/virtual_memory.h	(revision 41212)
+++ runtime/vm/virtual_memory.h	(working copy)
@@ -60,9 +60,14 @@
 
   static bool InSamePage(uword address0, uword address1);
 
-  // Truncate this virtual memory segment.
+  // Truncate this virtual memory segment. If try_unmap is false, the
+  // memory beyond the new end is still accessible, but will be returned
+  // upon destruction.
   void Truncate(intptr_t new_size, bool try_unmap = true);
 
+  // Commit a reserved memory area, so that the memory can be accessed.
+  bool Commit(uword addr, intptr_t size, bool is_executable);
+
  private:
   static VirtualMemory* ReserveInternal(intptr_t size);
 
@@ -76,9 +81,6 @@
       region_(region.pointer(), region.size()),
       reserved_size_(region.size()) { }
 
-  // Commit a reserved memory area, so that the memory can be accessed.
-  bool Commit(uword addr, intptr_t size, bool is_executable);
-
   MemoryRegion region_;
 
   // The size of the underlying reservation not yet given back to the OS.