Rename TestMojoMediaClient -> DefaultMojoMediaClient.

media/media_options.gni

Index: media/media_options.gni
diff --git a/media/media_options.gni b/media/media_options.gni
index 09e1cc2280e782389522e25842d89b666d109a95..f0faa1387c90be28cb3e550f4660315ca845f2f4 100644
--- a/media/media_options.gni
+++ b/media/media_options.gni
@@ -73,10 +73,6 @@ declare_args() {
   # the media pipeline and corresponding services will hosted in the selected
   # remote process (e.g. "utility" process, see |mojo_media_host|).
   enable_mojo_media = is_android || (is_chromecast && !is_cast_desktop_build)
-
-  # Enable the TestMojoMediaClient to be used in MojoMediaApplication. This is
-  # for testing only and will override the default platform MojoMediaClient.
-  enable_test_mojo_media_client = false
 }
 
 # Use a second declare_args() to pick up possible overrides of enable_mojo_media