[Video Capture Service] Improve UMA

This CL is part of the Mojo Video Capture work.
Design doc for dev work [1].
Design doc for launch [2].

After this CL we differentiate in the UMA logging between
enumeration-only usages of the service and usages that include actual capture.

BUG:584797,721812

[1] https://docs.google.com/a/chromium.org/document/d/1Qw7rw1AJy0QHXjha36jZNiEuxsxWslJ_X-zpOhijvI8/edit?usp=sharing
[2] https://docs.google.com/document/d/1RLlgEdvqRA_NQfSPMJLn5KR-ygVzZ2MRgIy9yd6CdFA/edit?usp=sharing

Change-Id: If6fd5fd1480edf18cce861f6c880d811a2f9a58c
Reviewed-on: https://chromium-review.googlesource.com/572220


Commit-Queue: Christian Fremerey <chfremer@chromium.org>
Reviewed-by: Emircan Uysaler <emircan@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488319}