Add back missing code for fullscreen change.

When fullscreen widget code was removed in crrev.com/406647a3 an
additional code path was accidentally removed. Add that code
path back in because it broken a callback that was necessary for
cast. It used it in the non-optimized fullscrene video for cast.

BUG=1144255

Change-Id: I6b3bdd2b20614a5995120434bd7d429119c0526d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2514076
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823526}