Add unmarkEndOfStream() to internal MediaSource interfaces
and rename endOfStream() to markEndOfStream().

This patch allows the MediaSourceBase to signal to a 
WebMediaSourceClient when the "end of stream" state is 
cancelled by some JavaScript call. The current code 
implicity detects the transition by watching for 
WebSourceBuffer.append() calls that happen after 
endOfStream(), but there are other situations where the 
transition happens and the WebMediaSourceClient isn't aware
of it but needs to be. This change removes the need for
implicit detection of the transition and will make it easier 
to fix bugs in theChromium-side of the MediaSource code.

BUG=259419
TEST=None. Can't test anything until Chromium-side changes land.

Review URL: https://chromiumcodereview.appspot.com/18807005

git-svn-id: svn://svn.chromium.org/blink/trunk@154276 bbb929c8-8fbe-4397-9dbb-9b2b20218538