Fix buffered range computation so it includes text tracks and conforms to the MSE spec.

BUG=230708,239506

Review URL: https://codereview.chromium.org/118793003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242628 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242628