Record UKM metrics for AMP subframe documents.

Change-Id: I81602ca1ddae82456fecc5a14e98f844093405bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1506001
Commit-Queue: Bryan McQuade <bmcquade@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638567}
4 files changed