Make sure WebRTC works without libvpx VP9 support.

webrtc/modules/video_coding/codecs/vp9/vp9.gyp

Index: webrtc/modules/video_coding/codecs/vp9/vp9.gyp
diff --git a/webrtc/modules/video_coding/codecs/vp9/vp9.gyp b/webrtc/modules/video_coding/codecs/vp9/vp9.gyp
index cd5201f8aaca1c9d04ea5a3de85a03cc9fb5c068..9124e5fad30eb0923563482c4f8f868590aeb94a 100644
--- a/webrtc/modules/video_coding/codecs/vp9/vp9.gyp
+++ b/webrtc/modules/video_coding/codecs/vp9/vp9.gyp
@@ -20,6 +20,21 @@
             '<(libvpx_dir)/libvpx.gyp:libvpx',
           ],
         }],
+        ['libvpx_build_vp9==1', {
+          'sources': [
+            'screenshare_layers.cc',
+            'screenshare_layers.h',
+            'vp9_frame_buffer_pool.cc',
+            'vp9_frame_buffer_pool.h',
+            'vp9_impl.cc',
+            'vp9_impl.h',
+          ],
+        }, {
+          'sources': [
+            'vp9_noop.cc',
+          ],
+        }
+        ],
       ],
       'dependencies': [
         '<(webrtc_root)/common_video/common_video.gyp:common_video',
@@ -28,12 +43,6 @@
       ],
       'sources': [
         'include/vp9.h',
-        'screenshare_layers.cc',
-        'screenshare_layers.h',
-        'vp9_frame_buffer_pool.cc',
-        'vp9_frame_buffer_pool.h',
-        'vp9_impl.cc',
-        'vp9_impl.h',
       ],
     },
   ],