Suppress JS ERROR as 'assignment to undeclared variable _myDwell'

syslog always said like a below:
Nov 22 04:56:42 localhost gnome-session[1555]: JS ERROR: !!!   WARNING: 'assignment to undeclared variable _myDwell'

This modification has suppress above JS ERROR.
Update extension.js
Merge pull request #2 from moshi/patch-1

Suppress JS ERROR as 'assignment to undeclared variable _myDwell'