Fix compilation if HAVE_WEBRTC_VIDEO is not defined.

talk/media/webrtc/webrtcmediaengine.cc

Index: talk/media/webrtc/webrtcmediaengine.cc
diff --git a/talk/media/webrtc/webrtcmediaengine.cc b/talk/media/webrtc/webrtcmediaengine.cc
index 31e5025a55c63a58bf1b541ae3d273de4688f56e..a014a0f4dd689ce88e460de9dec2433161545da6 100644
--- a/talk/media/webrtc/webrtcmediaengine.cc
+++ b/talk/media/webrtc/webrtcmediaengine.cc
@@ -29,13 +29,21 @@
 
 #include <algorithm>
 
+#ifdef HAVE_WEBRTC_VIDEO
 #include "talk/media/webrtc/webrtcvideoengine2.h"
+#else
+#include "talk/media/webrtc/nullwebrtcvideoengine.h"
+#endif
 #include "talk/media/webrtc/webrtcvoiceengine.h"
 
 namespace cricket {
 
 class WebRtcMediaEngine2
+#ifdef HAVE_WEBRTC_VIDEO
     : public CompositeMediaEngine<WebRtcVoiceEngine, WebRtcVideoEngine2> {
+#else
+    : public CompositeMediaEngine<WebRtcVoiceEngine, NullWebRtcVideoEngine> {
+#endif
  public:
   WebRtcMediaEngine2(webrtc::AudioDeviceModule* adm,
                      WebRtcVideoEncoderFactory* encoder_factory,