[Picture-in-Picture] Add UseCounter.

As part of the origin trial, we should record usage with UseCounter, as that
can be automatically monitored by the origin trials infrastructure.

Bug: 806249
Change-Id: I7978d33f57c9d31ecc2b18a410c66475f3971ddb
Reviewed-on: https://chromium-review.googlesource.com/1055396
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558324}
10 files changed