WebmMuxer is used to multiplex incoming encoded video
and audio into a webm live stream. It is only used from
content's MediaRecorderHandler.

This CL moves WebmMuxer to media/muxers where it
belongs, since it has nothing to do with capture/; that is
made even more relevant in a parallel CL
(http://crrev.com/1699553002) that isolates the said
media/capture.

This CL doesn't modify code (except a minor style
reformatting caught by the presubmit). Narrowed some
DEPS too.

BUG=584797

Review URL: https://codereview.chromium.org/1710713002

Cr-Commit-Position: refs/heads/master@{#376243}