Enable oilpan in modules/mediasource

Replacing transition types to real types.

BUG=340522

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=173593

Review URL: https://codereview.chromium.org/270253004

git-svn-id: svn://svn.chromium.org/blink/trunk@177275 bbb929c8-8fbe-4397-9dbb-9b2b20218538