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}
bool WebContentsImpl::IsWidgetForMainFrame(
    RenderWidgetHostImpl* render_widget_host) {
  return render_widget_host == GetMainFrame()->GetRenderWidgetHost();
}

bool WebContentsImpl::IsWidgetForMainFrame(
    RenderWidgetHostImpl* render_widget_host) {
  return render_widget_host == GetMainFrame()->GetRenderWidgetHost();
}
