Create JS executor and context on main thread w/ custom JSC

Reviewed By: javache

Differential Revision: D5165405

fbshipit-source-id: 35e5cfdf0518968c095ba54ec980d2fe376fa20e

Author: dcaspi

React/CxxBridge/RCTMessageThread.h

@@ -24,6 +24,7 @@ class RCTMessageThread : public MessageQueueThread {
   void runOnQueue(std::function<void()>&&) override;
   void runOnQueueSync(std::function<void()>&&) override;
   void quitSynchronous() override;
+  void setRunLoop(NSRunLoop *runLoop);
 
  private:
   void tryFunc(const std::function<void()>& func);

React/CxxBridge/RCTMessageThread.mm

@@ -94,5 +94,11 @@
   CFRunLoopStop(m_cfRunLoop);
 }
 
+void RCTMessageThread::setRunLoop(NSRunLoop *runLoop) {
+  CFRelease(m_cfRunLoop);
+  m_cfRunLoop = [runLoop getCFRunLoop];
+  CFRetain(m_cfRunLoop);
+}
+
 }
 }