MediaStream API: Introduce new track related APIs
As a step in the make track more stand-alone this patch intruduces a new
API and removes the MediaStream argument from the enable/disable calls.

BUG=294145

Review URL: https://codereview.chromium.org/75153005

git-svn-id: svn://svn.chromium.org/blink/trunk@162206 bbb929c8-8fbe-4397-9dbb-9b2b20218538