Fix/deflake tests in /chrome/browser/printing.

This is a prerequisite for switching WindowedNotificationObserver to use
base::RunLoop instead of deprecated MessageLoopRunner.

This CL is split out from https://crrev.com/c/982612.

This CL was uploaded by git cl split.

R=dpapad@chromium.org

Bug: 668707
Change-Id: I723a927eadbb422cff9165e159bfcb72412f6c5a
Reviewed-on: https://chromium-review.googlesource.com/1120499Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Alexander Semashko <ahest@yandex-team.ru>
Cr-Commit-Position: refs/heads/master@{#571620}

chrome/browser/printing/print_preview_dialog_controller_browsertest.cc

@@ -272,6 +272,11 @@ IN_PROC_BROWSER_TEST_F(PrintPreviewDialogControllerBrowserTest,
   chrome::Reload(browser(), WindowOpenDisposition::CURRENT_TAB);
   content::WaitForLoadStop(
       browser()->tab_strip_model()->GetActiveWebContents());
+  // When Widget::Close is called, a task is posted that will destroy the
+  // widget. Here the widget is closed when the navigation commits. Load stop
+  // may occur right after the commit, before the widget is destroyed.
+  // Execute pending tasks to account for this.
+  base::RunLoop().RunUntilIdle();
   ASSERT_TRUE(dialog_destroyed_observer.dialog_destroyed());
 
   // Try printing again.