Set updated_rect for frames generated by WindowCapturer implementationsw

Previous updated_rect wasn't set for frames generated by WindowCapturer
implementation. That makes them unustable with chromoting host that
uses update_rect. With that change the frames will always contain
updated_rect that coveras the whole frame.

Change by Ronak Vora <ronakvora@google.com>

R=wez@chromium.org

Review URL: https://webrtc-codereview.appspot.com/22079004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6912 4adac7df-926f-26a2-2b94-8c16560cd09d