[signin] Add histograms that track when the Gaia related headers are added or removed.

This CL does not solve the bug, but it adds instrumentation that could help us
understand if Chromium sends the Desktop Identity Consistency headers for
requests that are sent to Gaia.

Bug: 865941
Change-Id: I868394f7b23390b87494b26c49e2f8ac13a21795
Reviewed-on: https://chromium-review.googlesource.com/1145263
Commit-Queue: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577143}
6 files changed