Add base::UnguessableToken

mojo/common/test_common_custom_types.mojom

Index: mojo/common/test_common_custom_types.mojom
diff --git a/mojo/common/test_common_custom_types.mojom b/mojo/common/test_common_custom_types.mojom
index 88f03619f6a88313a599a1ad188ed6844a92f54f..b80842fe8b38b15fe71a1a2f7287b66924ea06d5 100644
--- a/mojo/common/test_common_custom_types.mojom
+++ b/mojo/common/test_common_custom_types.mojom
@@ -11,6 +11,11 @@ interface TestFilePath {
       => (mojo.common.mojom.FilePath out);
 };
 
+interface TestUnguessableToken {
+  BounceNonce(mojo.common.mojom.UnguessableToken in)
+      => (mojo.common.mojom.UnguessableToken out);
+};
+
 interface TestTime {
   BounceTime(mojo.common.mojom.Time time) => (mojo.common.mojom.Time time);
   BounceTimeDelta(mojo.common.mojom.TimeDelta time_delta)