Add check for if a native video track exist before creating a RTCVideoRenderer.

We have currently not implemented support for rendering video in new video tracks were the source is from a remote video track.
This check was accidentally removed in r252562.

BUG= 346003
R=joi@chromium.org, xians@chromium.org

Review URL: https://codereview.chromium.org/177363003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253107 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253107