Add an identity (id) to system drag & drop.

chrome/common/render_messages.h

Index: chrome/common/render_messages.h
diff --git a/chrome/common/render_messages.h b/chrome/common/render_messages.h
index e2a0aa03ab84194af53042e060287520b6b3ca94..7fa88de3c4973181a946ced2992153da7efad7e0 100755
--- a/chrome/common/render_messages.h
+++ b/chrome/common/render_messages.h
@@ -1539,6 +1539,7 @@ template <>
 struct ParamTraits<WebDropData> {
   typedef WebDropData param_type;
   static void Write(Message* m, const param_type& p) {
+    WriteParam(m, p.identity);
     WriteParam(m, p.url);
     WriteParam(m, p.url_title);
     WriteParam(m, p.file_extension);
@@ -1551,6 +1552,7 @@ struct ParamTraits<WebDropData> {
   }
   static bool Read(const Message* m, void** iter, param_type* p) {
     return
+      ReadParam(m, iter, &p->identity) &&
       ReadParam(m, iter, &p->url) &&
       ReadParam(m, iter, &p->url_title) &&
       ReadParam(m, iter, &p->file_extension) &&