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}
void WebContentsImpl::WebContentsTreeNode::SetFocusedWebContents(
    WebContentsImpl* web_contents) {
  DCHECK(!outer_web_contents())
      << "Only the outermost WebContents tracks focus.";
  focused_web_contents_ = web_contents;
}

void WebContentsImpl::WebContentsTreeNode::SetFocusedWebContents(
    WebContentsImpl* web_contents) {
  DCHECK(!outer_web_contents())
      << "Only the outermost WebContents tracks focus.";
  focused_web_contents_ = web_contents;
}
