Collect CPU frequency thermal throttling stats

This collects the max frequency every 30s and reports
it as a percentage of the unthrottled frequency.
The special value of 101% indicates that the CPU is
using turbo.

BUG=chromium:238890
TEST=ran manually, checked about:histograms

Change-Id: Ia1c8a2344b81b8274f9045b854d2e6d35cf49339
Reviewed-on: https://gerrit.chromium.org/gerrit/50387


Reviewed-by: Darin Petkov <petkov@chromium.org>
Commit-Queue: Luigi Semenzato <semenzato@chromium.org>
Tested-by: Luigi Semenzato <semenzato@chromium.org>