Remove the race condition between new process creation and context release (#12342)

* Remove the race condition between new process creation and old process releasing remote context

Previously there was a race condition where the getId() method would return the new context ID even
though the release was for the old context.  This changes it to send the "initial" context ID with
the release message to ensure there is no race.

* fetch context ID from remote in sandbox mode

atom/browser/api/atom_api_web_contents.h

@@ -89,6 +89,8 @@ class WebContents : public mate::TrackableObject<WebContents>,
   static void BuildPrototype(v8::Isolate* isolate,
                              v8::Local<v8::FunctionTemplate> prototype);
 
+  static int64_t GetIDForContents(content::WebContents* web_contents);
+
   // Notifies to destroy any guest web contents before destroying self.
   void DestroyWebContents(bool async);