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 OpenDevToolsWindow(const std::string& test_page) {
    ASSERT_TRUE(test_server()->Start());
    LoadTestPage(test_page);

    content::WindowedNotificationObserver observer(
        content::NOTIFICATION_LOAD_STOP,
        content::NotificationService::AllSources());
    inspected_rvh_ = GetInspectedTab()->GetRenderViewHost();
    window_ = DevToolsWindow::OpenDevToolsWindow(inspected_rvh_);
    observer.Wait();
  }

  void OpenDevToolsWindow(const std::string& test_page) {
    ASSERT_TRUE(test_server()->Start());
    LoadTestPage(test_page);

    content::WindowedNotificationObserver observer(
        content::NOTIFICATION_LOAD_STOP,
        content::NotificationService::AllSources());
    inspected_rvh_ = GetInspectedTab()->GetRenderViewHost();
    window_ = DevToolsWindow::OpenDevToolsWindow(inspected_rvh_);
    observer.Wait();
  }
