MediaStream API: Fix lifetime issues
After some recent refactorings some life-time assumptions are no longer true so in this patch I have added proper cleanup of a pointer.

Review URL: https://codereview.chromium.org/32363004

git-svn-id: svn://svn.chromium.org/blink/trunk@160182 bbb929c8-8fbe-4397-9dbb-9b2b20218538