Add UMA metric to track usage of sending a mousedown to select elements.

There is potential around the fast click library trapping touchend events
and generating mousedown events for android chrome.

Track the usage of this behaviour we are seeing.

BUG=642698

Review-Url: https://codereview.chromium.org/2303013002
Cr-Commit-Position: refs/heads/master@{#416193}
3 files changed