Update EME playback-multiple-sessions test to avoid flakiness

Update the test to not call generateRequest() for either stream
until both "onencrypted" events are received. This is to avoid
the occasional race condition where the first "message" event
arrives before the second "onencrypted" event, and thus messes
up the expected-output log comparison.

BUG=407993
TEST=modified test still pass

Review URL: https://codereview.chromium.org/658633002

git-svn-id: svn://svn.chromium.org/blink/trunk@183842 bbb929c8-8fbe-4397-9dbb-9b2b20218538