MediaStream should fire ended event when all tracks are ended

BUG=227485

Review URL: https://codereview.chromium.org/13776002
Patch from Li Yin <li.yin@intel.com>.

git-svn-id: svn://svn.chromium.org/blink/trunk@148133 bbb929c8-8fbe-4397-9dbb-9b2b20218538
7 files changed