LayoutManager.onTabStateInitialized creashes when creating Handler not in UI thread

The crashe is because TabPersistentStore.mergeState() kicks off an
AsyncTask to restoreTabs and restoreTabs creates a Handler in
LayoutManager.onTabStateInitialized(). But the Handler is not created in
UI thread whose Looper is not created.

Bug: 778447
Change-Id: Ibad4993471c461c0df61903025211488f4e50328
Reviewed-on: https://chromium-review.googlesource.com/746025
Commit-Queue: Lei Tian <ltian@google.com>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513244}

chrome/android/java/src/org/chromium/chrome/browser/tabmodel/TabPersistentStore.java

@@ -1188,7 +1188,11 @@ public class TabPersistentStore extends TabPersister {
     }
 
     private void onStateLoaded() {
-        if (mObserver != null) mObserver.onStateLoaded();
+        if (mObserver != null) {
+            // mergeState() starts an AsyncTask to call this and this calls
+            // onTabStateInitialized which should be called from the UI thread.
+            ThreadUtils.runOnUiThread( () -> mObserver.onStateLoaded() );
+        }
     }
 
     private void loadNextTab() {