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 DevToolsWindow::FrontendWebContentsObserver::AboutToNavigateRenderView(
    content::RenderViewHost* render_view_host) {
  content::DevToolsClientHost::SetupDevToolsFrontendClient(render_view_host);
}

void DevToolsWindow::FrontendWebContentsObserver::AboutToNavigateRenderView(
    content::RenderViewHost* render_view_host) {
  content::DevToolsClientHost::SetupDevToolsFrontendClient(render_view_host);
}
