Fix crash where the SpeechRecognitionBubble doesn't have a parent.

      This can happen when there isn't a location bar.

BUG=173746


Review URL: https://chromiumcodereview.appspot.com/12197015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@180590 0039d316-1c4b-4281-b951-d872f2087c98