WebWorkers: implement tear down and terminate()

Summary:
public
Previously we did no cleanup when the executor was torn down and didn't expose a way to tear down the worker from JS. Now we do.

Termination happens synchronously and waits the the worker's MessageQueueThread to finish and join.

Reviewed By: mhorowitz

Differential Revision: D2815240

fb-gh-sync-id: 786beb30d4d64556586b91727f32e379c667a965

ReactAndroid/src/main/java/com/facebook/react/bridge/queue/MessageQueueThread.java

@@ -43,4 +43,11 @@ public interface MessageQueueThread {
    * {@link AssertionError}) if the assertion fails.
    */
   void assertIsOnThread();
+
+  /**
+   * Quits this MessageQueueThread. If called from this MessageQueueThread, this will be the last
+   * thing the thread runs. If called from a separate thread, this will block until the thread can
+   * be quit and joined.
+   */
+  void quitSynchronous();
 }

ReactAndroid/src/main/java/com/facebook/react/bridge/queue/MessageQueueThreadImpl.java

@@ -101,6 +101,7 @@ public class MessageQueueThreadImpl implements MessageQueueThread {
    * Quits this queue's Looper. If that Looper was running on a different Thread than the current
    * Thread, also waits for the last message being processed to finish and the Thread to die.
    */
+  @Override
   public void quitSynchronous() {
     mIsFinished = true;
     mLooper.quit();