GitLab

Breaks out one of the most complex media/ targets into its
own BUILD.gn file for improved readibility and targetting.

BUG=none
TEST=media_unittests

Review URL: https://codereview.chromium.org/396793003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284314 0039d316-1c4b-4281-b951-d872f2087c98