Implement support for SourceBuffer.remove()

BUG=166465
TEST=http/tests/media/media-source/mediasource-remove.html

Review URL: https://chromiumcodereview.appspot.com/22483002

git-svn-id: svn://svn.chromium.org/blink/trunk@155644 bbb929c8-8fbe-4397-9dbb-9b2b20218538