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::OnFrameTreeNodeDestroyed(
    FrameTreeNode* node) {
  DCHECK_EQ(outer_contents_frame_tree_node_id_, node->frame_tree_node_id())
      << "WebContentsTreeNode should only receive notifications for the "
         "FrameTreeNode in its outer WebContents that hosts it.";
  delete current_web_contents_;  // deletes |this| too.
}

void WebContentsImpl::WebContentsTreeNode::OnFrameTreeNodeDestroyed(
    FrameTreeNode* node) {
  DCHECK_EQ(outer_contents_frame_tree_node_id_, node->frame_tree_node_id())
      << "WebContentsTreeNode should only receive notifications for the "
         "FrameTreeNode in its outer WebContents that hosts it.";
  delete current_web_contents_;  // deletes |this| too.
}
