Speculative fix for NewTabPageTest#testArticleExpandableHeaderAppearance

Bug: 829377
Change-Id: Icf768d451f9967d954cf0ba40ff42a04c5dbe44d
Reviewed-on: https://chromium-review.googlesource.com/1011389Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Becky Zhou <huayinz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550702}

chrome/android/javatests/src/org/chromium/chrome/browser/ntp/NewTabPageTest.java

@@ -632,8 +632,9 @@ public class NewTabPageTest {
         RecyclerView recyclerView = ntp.getNewTabPageView().getRecyclerView();
         NewTabPageAdapter adapter = (NewTabPageAdapter) recyclerView.getAdapter();
         RecyclerViewTestUtils.waitForStableRecyclerView(recyclerView);
-        View view = recyclerView.findViewHolderForAdapterPosition(
-                adapter.getFirstHeaderPosition()).itemView;
+        View view = ThreadUtils.runOnUiThreadBlocking(
+                () -> recyclerView.findViewHolderForAdapterPosition(
+                        adapter.getFirstHeaderPosition()).itemView);
 
         // Check header is expanded.
         mRenderTestRule.render(view, "expandable_header_expanded");