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The old Symantec PKI will be fully distrusted in M70
(https://security.googleblog.com/2018/03/distrust-of-symantec-pki-immediate.html). We
already have DevTools console messages in place for affected resources, and this
CL adds UseCounters so that we know how many page loads will be affected.

The counters give us:
- % of page visits with an affected main frame main resource
- % of page visits with an afected subframe main resource
- % of page visits with an affected subresource

Note that these buckets can overlap, for example a single page visit can
increment all 3 counters.

Bug: 853868
Change-Id: I7ae103efcc47a3e4e381b35c6803ef2d8ed4c7fb
Reviewed-on: https://chromium-review.googlesource.com/1107841
Commit-Queue: Emily Stark <estark@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569282}