Prefixed MediaSource: Clarify durationchange+readyState verification

Per HTMLMediaElement spec, the element should be in at least
HAVE_METADATA readyState once duration is known. This change clarifies
the verification to the prefixed MSE API that 'durationchange' event
delivery doesn't occur while the element is in HAVE_NOTHING readyState.
Before this change, expectation mismatch would have been detected, and
test would have failed. This change makes it clear why the test might
fail, also assisting port to unprefixed MSE test harness in future.

R=acolwell@chromium.org
BUG=167688
TEST=updated layout test still passes locally on linux Chromium

Review URL: https://codereview.chromium.org/161363002

git-svn-id: svn://svn.chromium.org/blink/trunk@167555 bbb929c8-8fbe-4397-9dbb-9b2b20218538