media: Convert mojo media defines to buildflags

media/gpu/BUILD.gn

Index: media/gpu/BUILD.gn
diff --git a/media/gpu/BUILD.gn b/media/gpu/BUILD.gn
index 96fb0c882541fa06b39a3045854b72edcf1f991c..098b9cd129496055fabbd0bade8f2823d5757f84 100644
--- a/media/gpu/BUILD.gn
+++ b/media/gpu/BUILD.gn
@@ -204,6 +204,8 @@ component("gpu") {
       "surface_texture_gl_owner.h",
     ]
 
+    deps += [ "//media/mojo:features" ]
+
     if (enable_webrtc) {
       deps += [ "//third_party/libyuv" ]
       sources += [
@@ -224,7 +226,6 @@ component("gpu") {
     # media service. See http://crbug.com/522298
     if (mojo_media_host == "gpu") {
       deps += [ "//media:cdm_manager" ]
-      configs += [ "//media/mojo/services:mojo_media_config" ]
     }
   }