[Messages] Fix NPE on Mediator#resumeQueue

Reorder the initializaton of dispather and mediator so that
mediator can receive a correctly initialized dispatcher

Bug: 1144644
Change-Id: I8bc63b2444c32a4e9e8f3459a4a3ea15fe506ca8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2514772
Auto-Submit: Lijin Shen <lazzzis@google.com>
Reviewed-by: Theresa  <twellington@chromium.org>
Commit-Queue: Theresa  <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823321}

chrome/android/java/src/org/chromium/chrome/browser/ui/RootUiCoordinator.java

@@ -392,10 +392,10 @@ public class RootUiCoordinator
             MessageContainer container = mActivity.findViewById(R.id.message_container);
             mMessageContainerCoordinator =
                     new MessageContainerCoordinator(container, getBrowserControlsManager());
+            mMessageDispatcher = MessagesFactory.createMessageDispatcher(container);
             mMessageQueueMediator = new ChromeMessageQueueMediator(
                     mActivity.getBrowserControlsManager(), mMessageContainerCoordinator,
                     mActivity.getFullscreenManager(), mMessageDispatcher);
-            mMessageDispatcher = MessagesFactory.createMessageDispatcher(container);
             mMessageDispatcher.setDelegate(mMessageQueueMediator);
             MessagesFactory.attachMessageDispatcher(
                     mActivity.getWindowAndroid(), mMessageDispatcher);