MediaStream API: Adding ExtraData functionality to WebMediaStreamTrack
As part of the work to make Tracks more standalone we need ExtraData functionality in WebMediaStreamTrack.

BUG=294145

Review URL: https://codereview.chromium.org/24535002

git-svn-id: svn://svn.chromium.org/blink/trunk@158720 bbb929c8-8fbe-4397-9dbb-9b2b20218538