Remove prefixed Media Source API.

Removing prefixed Media Source API as discussed on blink-dev.
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/J_VC_Mq9wvg

BUG=363444

Review URL: https://codereview.chromium.org/237913004

git-svn-id: svn://svn.chromium.org/blink/trunk@172098 bbb929c8-8fbe-4397-9dbb-9b2b20218538