Making the Mojo Channel Messages legacy mode dynamic.

mojo/edk/system/BUILD.gn

Index: mojo/edk/system/BUILD.gn
diff --git a/mojo/edk/system/BUILD.gn b/mojo/edk/system/BUILD.gn
index b0acf237b36121c2878133f7f574780df9c22996..d519387f29aff36ab34d6854aa30fd3ccce1a506 100644
--- a/mojo/edk/system/BUILD.gn
+++ b/mojo/edk/system/BUILD.gn
@@ -110,12 +110,6 @@ component("system") {
     ]
   }
 
-  # Use target_os == "chromeos" instead of is_chromeos because we need to
-  # build NaCl targets (i.e. IRT) for ChromeOS the same as the rest of ChromeOS.
-  if (is_android || target_os == "chromeos") {
-    defines += [ "MOJO_EDK_LEGACY_PROTOCOL" ]
-  }
-
   allow_circular_includes_from = [ "//mojo/edk/embedder" ]
 }