Use an external-only VideoRenderModule in Call.

The default render module instantiated from inside VideoEngine if none
exists instantiates platform-specific code. Call only uses external
rendering, so this is an unneccessary overhead.

BUG=1667
R=mflodman@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/39069004

Cr-Commit-Position: refs/heads/master@{#8346}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8346 4adac7df-926f-26a2-2b94-8c16560cd09d