[Clank] Add functions to clear offline media licenses.

Clear media licenses will remove the session data from persist storage.
If all the licenses are removed for the origin, the origin will be
unprovisioned, a.k.a cert is removed.

In both cases, the license file is untouched because we can't get the
license release response from license server.

The code will be connected to UI in next diff.

BUG=493521
TEST=Clear all the licenses -> cert file is removed.
TEST=Clear some licenses -> offline video fails to play.
[email protected], [email protected]

Change-Id: Ia4b418ab414c01f90432573118ea0e2e05ef6957
Reviewed-on: https://chromium-review.googlesource.com/602671
Commit-Queue: Yuchen Liu <[email protected]>
Reviewed-by: John Rummell <[email protected]>
Cr-Commit-Position: refs/heads/master@{#496158}
11 files changed