Add use counters for :focus and :focus-visible.

The goal of this change is to track developer uptake of :focus-visible,
once it ships from behind a flag. We also want to monitor the usage
of :focus. This data will help us explore possibly switching the UA stylesheet
over to using :focus-visible someday.


R=aboxhall@chromium.org

Bug: 817199
Change-Id: I257ab590f03d28831ec517b67f42e6f86a156ee9
Reviewed-on: https://chromium-review.googlesource.com/954102
Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Takayoshi Kochi <kochi@chromium.org>
Reviewed-by: Luna Lu <loonybear@chromium.org>
Reviewed-by: Rick Byers <rbyers@chromium.org>
Commit-Queue: Rob Dodson <robdodson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542279}
4 files changed