Support more configurability in GN toolchains

tools/gn/example/hello_shared.cc

Index: tools/gn/example/hello_shared.cc
diff --git a/mojo/services/public/interfaces/surfaces/surface_id.mojom b/tools/gn/example/hello_shared.cc
similarity index 69%
copy from mojo/services/public/interfaces/surfaces/surface_id.mojom
copy to tools/gn/example/hello_shared.cc
index 5a7686da33112b7d5d64204c42d5f6da6efadc39..58be84c27b4782d13ecc286203f7973b6953488e 100644
--- a/mojo/services/public/interfaces/surfaces/surface_id.mojom
+++ b/tools/gn/example/hello_shared.cc
@@ -2,10 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-module mojo {
-
-struct SurfaceId {
-  uint64 id;
-};
+#include "hello_shared.h"
 
+const char* GetSharedText() {
+  return "world";
 }