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
string16 DevToolsConfirmInfoBarDelegate::GetMessageText() const {
  return message_;
}

string16 DevToolsConfirmInfoBarDelegate::GetMessageText() const {
  return message_;
}
