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

components/gcm_driver/instance_id/instance_id_android.h

Index: components/gcm_driver/instance_id/instance_id_android.h
diff --git a/components/gcm_driver/instance_id/instance_id_android.h b/components/gcm_driver/instance_id/instance_id_android.h
index a11529604142cc774d0a96cf0e208ef3e6386a97..9fe1856493f23ae4605a2e0310118496891e70c0 100644
--- a/components/gcm_driver/instance_id/instance_id_android.h
+++ b/components/gcm_driver/instance_id/instance_id_android.h
@@ -27,6 +27,11 @@ class InstanceIDAndroid : public InstanceID {
   // Register JNI methods.
   static bool RegisterJni(JNIEnv* env);
 
+  // Call this (with block=true) in SetUp of all tests using Instance IDs except
+  // content_browsertests and layout tests, as those two test suites use
+  // NestedMessagePumpAndroid which makes this unnecessary.

[Peter Beverloo, 2016/04/15 00:00:17]

No need to make this specific to environments, that's bound to go out of date.

  // Tests depending on InstanceID in an environment where no nested message
  // loops are available should enable blocking on asynchronous operations
  // using this mechanism.

[johnme, 2016/04/15 16:00:43]

Done (reworded).

+  static void SetBlockOnAsyncTasksForTesting(bool block);
+
   InstanceIDAndroid(const std::string& app_id);
   ~InstanceIDAndroid() override;