Make sure WebRTC works without libvpx VP9 support.

webrtc/build/common.gypi

Index: webrtc/build/common.gypi
diff --git a/webrtc/build/common.gypi b/webrtc/build/common.gypi
index 5335cc8b16a8ffe0f679977fe28435ad11e0185c..15aa19155f3d3568fbf31966fcf10b7316c39639 100644
--- a/webrtc/build/common.gypi
+++ b/webrtc/build/common.gypi
@@ -102,6 +102,7 @@
     'build_libjpeg%': 1,
     'build_libsrtp%': 1,
     'build_libvpx%': 1,
+    'libvpx_build_vp9%': 1,
     'build_libyuv%': 1,
     'build_openmax_dl%': 1,
     'build_opus%': 1,
@@ -447,6 +448,11 @@
           'WEBRTC_INCLUDE_INTERNAL_AUDIO_DEVICE',
         ],
       }],
+      ['libvpx_build_vp9==0', {
+        'defines': [
+          'RTC_DISABLE_VP9',
+        ],
+      }],
     ], # conditions
     'direct_dependent_settings': {
       'conditions': [