Add Go build support for GN.

mojo/public/go/mojo/system/core.go

Index: mojo/public/go/mojo/system/core.go
diff --git a/mojo/services/public/interfaces/view_manager/view_manager_constants.mojom b/mojo/public/go/mojo/system/core.go
similarity index 72%
copy from mojo/services/public/interfaces/view_manager/view_manager_constants.mojom
copy to mojo/public/go/mojo/system/core.go
index 5d1380542cbaba02a3a741cd67d2d0e82286ad4d..8049d37b3bb9974771109712cf4eee60177000ef 100644
--- a/mojo/services/public/interfaces/view_manager/view_manager_constants.mojom
+++ b/mojo/public/go/mojo/system/core.go
@@ -2,11 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-module mojo {
-
-enum OrderDirection {
-  ABOVE = 1,
-  BELOW,
-};
+package system;
 
+type Core interface {
+	GetTimeTicksNow() int64
 }