fix build when ffmpeg, libvpx and libwebm are disabled

media/BUILD.gn

Index: media/BUILD.gn
diff --git a/media/BUILD.gn b/media/BUILD.gn
index 52af4930d44f187a9537058826ed5832b25b4dc5..2956b2a6a54f82ca2abbb00814fee9719e3a2de2 100644
--- a/media/BUILD.gn
+++ b/media/BUILD.gn
@@ -20,6 +20,9 @@ config("media_config") {
   if (!media_use_ffmpeg) {
     defines += [ "MEDIA_DISABLE_FFMPEG" ]
   }
+  if (!media_use_libwebm) {
+    defines += [ "MEDIA_DISABLE_LIBWEBM" ]
+  }
   if (current_cpu == "arm" && arm_use_neon) {
     defines += [ "USE_NEON" ]
   }
@@ -49,6 +52,9 @@ config("media_dependent_config") {
   if (!media_use_ffmpeg) {
     defines += [ "MEDIA_DISABLE_FFMPEG" ]
   }
+  if (!media_use_libwebm) {
+    defines += [ "MEDIA_DISABLE_LIBWEBM" ]
+  }
   if (is_win) {
     ldflags = [
       "/DELAYLOAD:mf.dll",