GN: Implement video_engine, video_capture and video_render.

Also add more from common.gypi to webrtc.gni.

These GN configs are based on GYP files in r6997.

BUG=3441
TEST=Trybots and local compile using:
gn gen out/Default --args="build_with_chromium=false" && ninja -C out/Default
gn gen out/Default --args="build_with_chromium=false is_debug=true" && ninja -C out/Default

Passed compile from a Chromium checkout with src/third_party/webrtc linked to the webrtc/ dir of a checkout with this patch applied.

R=brettw@chromium.org, glaznev@webrtc.org, mflodman@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/18709004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6999 4adac7df-926f-26a2-2b94-8c16560cd09d