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Now that there is support to count the number of active MediaKey
and MediaKeySession objects, update the tests to check the count
directly (there should be no such objects at the start of the test,
and if there are then something else is running and the counts
will be wrong anyway). By converting these tests to promise tests,
we get better logging if the test fails.

BUG=445324
TEST=updated tests pass

Review-Url: https://codereview.chromium.org/2618603002
Cr-Commit-Position: refs/heads/master@{#443113}