Start dismantling blink::MediaStreamCenter

This is the first of 'n' CLs that dismantle both
MediaStreamCenter classes in content and blink.
It is part of phase 4.7 in the design doc [1].

[1] https://docs.google.com/document/d/1rHJGi1U72qZsOGIctIf7GKRaY8d7BgtVW8MaMYumzYY/

As a first step, this CL starts by embedding the blink::MediaStreamCenter
methods used by other blink/renderer/platform/mediastream classes into
their respective callsides.

This steps makes further work easier, since the remaining
users of the blink::MediaStreamCenter APIs are in blink/renderer/modules/.

The methods being moved here are:

- MediaStreamCenter::SetContentHint
- MediaStreamCenter::GetSourceSettings

BUG=704136
R=guidou@chromium.org, haraken@chromium.org

Change-Id: I75a8487c4fca0838f49b92311f5d80ec4e929435
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1726569
Auto-Submit: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#682573}
7 files changed