Make ready to unprefix getUserMedia, and counters
This CL adds an unprefixed getUserMedia on navigator,
behind the same flag as the promise-based getUserMedia.
It also counts all three variants.
BUG=
607439
Committed:
https://crrev.com/489f502a4d2d39665eb58158dc5df313ee2dbf15
Cr-Commit-Position: refs/heads/master@{#399156}