Bug 667652: Add a JS_FRIEND_API to get at the current offset in a structured clone buffer. r=jorendorff.

js/src/jsclone.h

@@ -46,6 +46,9 @@
 #include "jsvector.h"
 #include "jsvalue.h"
 
+JS_FRIEND_API(uint64_t)
+js_GetSCOffset(JSStructuredCloneWriter* writer);
+
 namespace js {
 
 bool
@@ -73,6 +76,8 @@ struct SCOutput {
 
     bool extractBuffer(uint64_t **datap, size_t *sizep);
 
+    uint64_t count() { return buf.length(); }
+
   private:
     JSContext *cx;
     js::Vector<uint64_t> buf;