Add lifetime tests for MediaKeySession

Since MediaKeySessions are ActiveDOMObjects and can be counted, adding tests
to verify that garbage collection frees the sessions when they are no longer
needed. One test simply uses JS references to keep the sessions around, while
the other calls release() on the session to free them.

BUG=341567
TEST=new tests pass

Review URL: https://codereview.chromium.org/209103002

git-svn-id: svn://svn.chromium.org/blink/trunk@170341 bbb929c8-8fbe-4397-9dbb-9b2b20218538