Allow MediaDrmStorage::Initialize() to pass empty UnguessableTokens

base::UnguessableToken doesn't allow serialization of Null(). So make
the |origin_id| parameter of MediaDrmStorage::Initialize() optional.

BUG=917527
TEST=new unit_tests pass

Change-Id: I21e374b42e670220ea11166c77651f0d9c00ae12
Reviewed-on: https://chromium-review.googlesource.com/c/1475902
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Commit-Queue: John Rummell <jrummell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634306}
11 files changed