WebUI: Update MockTimer to an es6 class, autogenerate mock_timer.m.js

In preparation for autogenerating more cr-elements tests in
https://crrev.com/c/1752252:
- Update MockTimer to be an es6 class
- Autogenerate mock_timer.m.js and remove the checked-in version

Bug: 965770
Change-Id: Ia7c66407bb25a8b41ea531847eb64c0617f3495b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1754266
Reviewed-by: John Lee <johntlee@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686988}
4 files changed