DevTools: handle devtools renderer unresponsiveness during beforeunload event interception

This patch fixes the crash which happenes under the following conditions:
1. DevTools window is in undocked state
2. DevTools renderer is unresponsive
3. User attempts to close inspected page

BUG=322380

Review URL: https://codereview.chromium.org/84883002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237611 0039d316-1c4b-4281-b951-d872f2087c98
  void CloseDevToolsWindow() {
    DevToolsManager* devtools_manager = DevToolsManager::GetInstance();
    content::WindowedNotificationObserver close_observer(
        content::NOTIFICATION_WEB_CONTENTS_DESTROYED,
        content::Source<content::WebContents>(window_->web_contents()));
    devtools_manager->CloseAllClientHosts();
    close_observer.Wait();
  }

  void CloseDevToolsWindow() {
    DevToolsManager* devtools_manager = DevToolsManager::GetInstance();
    content::WindowedNotificationObserver close_observer(
        content::NOTIFICATION_WEB_CONTENTS_DESTROYED,
        content::Source<content::WebContents>(window_->web_contents()));
    devtools_manager->CloseAllClientHosts();
    close_observer.Wait();
  }
