Simplify WMPI::duration() MSE path
In MSE case, blink now consults MediaSource directly for duration,
and WMPI::duration() is only used to support maxTimeSeekable().
In this case, exact user-specified (double) duration available from
ChunkDemuxer is unnecessary, and we can use the (TimeDelta) duration
maintained by Pipeline.
R=acolwell@chromium.org
TEST=media unit and layout tests pass locally on chromium linux
Review URL: https://chromiumcodereview.appspot.com/
17448015
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@207551 0039d316-1c4b-4281-b951-d872f2087c98