MediaStream API: Add a stop method to MediaStreamTrack
First step in a series of patches that will move the stop functionality
from MediaStream to MediaStreamTrack.

BUG=294132

Review URL: https://codereview.chromium.org/24182006

git-svn-id: svn://svn.chromium.org/blink/trunk@158708 bbb929c8-8fbe-4397-9dbb-9b2b20218538