navigator.vibrate() crash avoidance when in a detached state.

If the frame isn't present, gracefully return.

R=mvanouwerkerk@chromium.org,haraken@chromium.org
BUG=375004

Review URL: https://codereview.chromium.org/298683002

git-svn-id: svn://svn.chromium.org/blink/trunk@174380 bbb929c8-8fbe-4397-9dbb-9b2b20218538