Add MojoVideoDecoder.

media/media_options.gni

Index: media/media_options.gni
diff --git a/media/media_options.gni b/media/media_options.gni
index 40bfc89b85502ce39a982c91065af37bf588d6b9..cedcdf7132793b16af8af32d28b0a981f2d924e6 100644
--- a/media/media_options.gni
+++ b/media/media_options.gni
@@ -85,12 +85,12 @@ declare_args() {
   # A list of mojo media services that should be used in the media pipeline.
   # Must not be empty if |enable_mojo_media| is true.
   # Valid entries in the list are:
-  # - "renderer": Use mojo based media Renderer service.
-  # - "cdm": Use mojo based Content Decryption Module.
-  # - "audio_decoder": Use mojo based audio decoder in the default media
+  # - "renderer": Use mojo-based media Renderer service.
+  # - "cdm": Use mojo-based Content Decryption Module.
+  # - "audio_decoder": Use mojo-based audio decoder in the default media
+  #                    Renderer. Cannot be used with the mojo Renderer above.
+  # - "video_decoder": Use mojo-based video decoder in the default media
   #                    Renderer. Cannot be used with the mojo Renderer above.
-  # TODO(xhwang): Support mojo base video decoders here.
-  # See http://crbug.com/542910 and http://crbug.com/522298
   mojo_media_services = []
 
   # The process to host the mojo media application.