Ignore fake mousemove event triggered by Chrome

These happen due to certain DOM changes. It's a hack in Chrome's
code and it seems to have gotten worse with a recent Chrome update,
causing this bug. Thankfully the fake event can be identified easily.

Change-Id: I1688054f5d57147a445db5116a6aafeb1f49ddc4
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/989