Update HTMLMediaElement.buffered behavior for MediaSource to match the spec.

This moves the cross SourceBuffer intersection computation into Blink so that
MediaPlayer implementations don't have to deal with it. This change also fixes
a bug with the computation of ranges for demuxed content after endOfStream()
is called.

BUG=259419
TEST=http/tests/media/media-source/mediasource-buffered.html

Review URL: https://chromiumcodereview.appspot.com/18755004

git-svn-id: svn://svn.chromium.org/blink/trunk@154238 bbb929c8-8fbe-4397-9dbb-9b2b20218538