Make ui::test::EventGenerator work with PointerEvents in mash

ui/aura/env.h

Index: ui/aura/env.h
diff --git a/ui/aura/env.h b/ui/aura/env.h
index b6b1b2c04c8ed57c0ab3f3ad470c21de20552c5f..b9b4d77005e6289fc7de07859d08307372084ebe 100644
--- a/ui/aura/env.h
+++ b/ui/aura/env.h
@@ -147,6 +147,9 @@ class AURA_EXPORT Env : public ui::EventTarget,
   // This is not const for tests, which may share Env across tests and so needs
   // to reset the value.
   Mode mode_;
+
+  // Intentionally not exposed publicly. Someday we might want to support

[James Cook, 2017/03/16 19:13:32]

I talked to Scott and he didn't want to expose this publicly in production but
said he was OK with exposing it for tests.

+  // multiple WindowTreeClients. Use EnvTestHelper in tests.
   WindowTreeClient* window_tree_client_ = nullptr;
 
   base::ObserverList<EnvObserver> observers_;