Cast: Wiring up the asyc initialization with the video encoders.

This is a follow up cl to complete the async intialization.

Review URL: https://codereview.chromium.org/151103004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249576 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249576