Fix WebMediaStreamTrack::ExtraData::owner()
Adding the missing initialization.

NOTRY=true
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/138053003

git-svn-id: svn://svn.chromium.org/blink/trunk@165069 bbb929c8-8fbe-4397-9dbb-9b2b20218538