This CL fixes compiler / linker errors that occur if HAVE_WEBRTC_VIDEO is not defined and introduces a new class NullWebRtcVideoEngine to use in that case. BUG= TEST=remove define HAVE_WEBRTC_VIDEO from talk/build/common.gypi, run gclient runhooks and compile Review URL: https://codereview.webrtc.org/1621453005 Cr-Commit-Position: refs/heads/master@{#11387}