Add UKM to note whether an AMP document was loaded in the main frame.

Bug: 926228
Change-Id: Iefa96a8d283753a1dfd580f3d09af659420275f4
Reviewed-on: https://chromium-review.googlesource.com/c/1447017
Commit-Queue: Annie Sullivan <sullivan@chromium.org>
Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630833}
11 files changed