If JavaScript shows a dialog, cause the page to lose fullscreen.

BUG=670135, 550017, 726761, 728276

Review-Url: https://codereview.chromium.org/2906133004
Cr-Commit-Position: refs/heads/master@{#478884}
  TestJavaScriptDialogManager() : message_loop_runner_(new MessageLoopRunner) {}

  TestJavaScriptDialogManager() : message_loop_runner_(new MessageLoopRunner) {}
