Add fake for InstanceIDWithSubtype.java, in order to re-use unit test

components/gcm_driver/instance_id/android/BUILD.gn

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/android/rules.gni")
 
 # GYP version: components/gcm_driver.gypi:instance_id_driver_jni_headers
 generate_jni("jni_headers") {
   sources = [
     "java/src/org/chromium/components/gcm_driver/instance_id/InstanceIDWithSubtype.java",
   ]
   jni_package = "components/gcm_driver/instance_id"
 }
 
 # GYP version: components/gcm_driver.gypi:instance_id_driver_java
 android_library("instance_id_driver_java") {
   deps = [
     "//base:base_java",
     google_play_services_library,
   ]
 
-  java_files = [ "java/src/org/chromium/components/gcm_driver/instance_id/InstanceIDWithSubtype.java" ]
+  java_files = [
+    "java/src/org/chromium/components/gcm_driver/instance_id/FakeInstanceIDWithSubtype.java",
+    "java/src/org/chromium/components/gcm_driver/instance_id/InstanceIDWithSubtype.java",
+  ]
 }