MediaStream API: Refactor the MediaStreamTrack creation code
Apart from the refactoring the code is also prepared for the big track
behavioral change coming up.

BUG=294145

Review URL: https://codereview.chromium.org/72043004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235997 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235997