Add PARSING_MEDIA_SEGMENT layout test for SourceBuffer mode

Adds layout test to verify that attempt to set
SourceBuffer's append mode fails when enough data has been
appended for the buffer to be in the middle of parsing a
media segment.
Also, removes default WebSourceBuffer::setMode()
implementation now that Chromium WSBI::setMode() has landed.

R=acolwell@chromium.org,jamesr@chromium.org
BUG=249422
TEST=All mediasource layout tests pass locally on Linux

Review URL: https://codereview.chromium.org/136523004

git-svn-id: svn://svn.chromium.org/blink/trunk@165100 bbb929c8-8fbe-4397-9dbb-9b2b20218538