Verify tighter PARSING_MEDIA_SEGMENT entrance constraint

Updates MSE SourceBuffer mode layout test to verify that
setting the mode fails when half of a media segment is
appended instead of much more. Tight conformance on a
per-parser-basis is left to WebSourceBuffer
implementations and their unit tests.
Also adds similar verification for SourceBuffer timestampOffset.

R=acolwell@chromium.org
BUG=334127
TEST=The updated layout test passes locally on Linux.

Review URL: https://codereview.chromium.org/131763005

git-svn-id: svn://svn.chromium.org/blink/trunk@165541 bbb929c8-8fbe-4397-9dbb-9b2b20218538