[ReactNative] Allow bridge modules to run on the JavaScript thread

React/Base/RCTBridgeModule.h

@@ -17,6 +17,16 @@
  */
 typedef void (^RCTResponseSenderBlock)(NSArray *response);
 
+/**
+ * This constant can be returned from +methodQueue to force module
+ * methods to be called on the JavaScript thread. This can have serious
+ * implications for performance, so only use this if you're sure it's what
+ * you need.
+ *
+ * NOTE: RCTJSThread is not a real libdispatch queue
+ */
+extern const dispatch_queue_t RCTJSThread;
+
 /**
  * Provides the interface needed to register a bridge module.
  */