Improve reliability of EME tests

The logs for these EME tests expect all the "onencrypted" events to
occur before the "message" event is received. Modify the tests to
create the session on the second event rather than the first, to avoid
the "message" event occasionally happening before the second
"onencrypted" event".

BUG=407993
TEST=modified tests still pass

Review URL: https://codereview.chromium.org/634243005

git-svn-id: svn://svn.chromium.org/blink/trunk@183833 bbb929c8-8fbe-4397-9dbb-9b2b20218538