Update off-canvas menu icon to be visible on Android 2.3 devices

As detailed on CanIUse:
"Android 2.3 default browser doesn't seem to appreciate 0px values. e.g. -webkit-box-shadow: 5px 1px 0px #f04e29; doesn't work, but -webkit-box-shadow: 5px 1px 1px #f04e29 does!"
http://caniuse.com/#search=box-shadow

This is the case on Android 2.3 browsers (i.e. a tested Samsung Galaxy S2). Modifying the 0 value to 1px only for the -webkit prefixed box-shadow solves the issue without causing visual degradation.
Merge pull request #3828 from johnbacon/patch-4

Update off-canvas menu icon to be visible on Android 2.3 devices