Fix Flaky Mouse latency test.

There are some timing issues that are tricky to get right for cases
where rendering is scheduled due to mouse input. Fix the flake by
preventing rendering from being scheduled.

Mouse down was triggering text selection logic, which scheduled
rendering. To prevent this, use user-select:none.

BUG=728486

Review-Url: https://codereview.chromium.org/2923623002
Cr-Commit-Position: refs/heads/master@{#477037}