Fix a test code memory leak in MediaStreamVideoSourceTest.DeliverVideoFrame.

Without the change the VideoCapturer will leak.
http://build.chromium.org/p/chromium.memory.fyi/builders/Linux%20Tests%20%28valgrind%29%281%29/builds/31136/steps/memory%20test%3A%20content/logs/stdio

BUG=334871
R=wjia@chromium.org

Review URL: https://codereview.chromium.org/139883005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245037 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245037