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When a subresource fails to load with
net::ERR_CERTIFICATE_TRANSPARENCY_REQUIRED, increment a UseCounter for the
document. This gives an upper-bound estimate on the percent of page loads with
user-visible CT-induced breakage. (A subresource failing to load might or might
not be user-visible breakage, which is why it's an upper bound.)

This metric will be useful for evaluating user impact as we roll out more
Certificate Transparency enforcement.

Note that this does not yet count subframe errors, due to some larger
PlzNavigate-related problems with how iframe errors are sent to
renderers. (https://crbug.com/750901)

Bug: 772534
Change-Id: Ib2367aea4556e355c9c4b3406199c177b0a6be79
Reviewed-on: https://chromium-review.googlesource.com/798639
Commit-Queue: Emily Stark <estark@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520868}