Speed up quoting of JSON strings by allocating a string that is big enough...

src/spaces.h

Index: src/spaces.h
===================================================================
--- src/spaces.h	(revision 5937)
+++ src/spaces.h	(working copy)
@@ -1643,6 +1643,11 @@
 
   virtual bool ReserveSpace(int bytes);
 
+  // Resizes a sequential string which must be the most recent thing that was
+  // allocated in new space.
+  template <typename StringType>
+  inline void ShrinkStringAtAllocationBoundary(String* string, int len);
+
 #ifdef ENABLE_HEAP_PROTECTION
   // Protect/unprotect the space by marking it read-only/writable.
   virtual void Protect();