Propagate media muted state to WebContents

Propagate media muted state to WebContents, cache it in WebContentsImpl
and make it accessible through GetMediaMuted() in C++.

BUG=729998

Change-Id: I7e6f5162db8045a02b88055ef262ec5c4e3d3913
Reviewed-on: https://chromium-review.googlesource.com/528152
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#480449}
14 files changed