Add UMA histogram for Expect-CT header processing

This CL records a histogram whenever an Expect-CT header is processed. This
tells us how many connections serve Expect-CT headers and how many parse
successfully.

Bug: 679012
Change-Id: I4cc8929a440e8f3508db53c424594afab8a7c318
Reviewed-on: https://chromium-review.googlesource.com/690705
Reviewed-by: Matt Mueller <mattm@chromium.org>
Reviewed-by: Emily Stark <estark@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Emily Stark <estark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505689}
3 files changed