aura: Introduce EventInjector.

ui/aura/env.h

Index: ui/aura/env.h
diff --git a/ui/aura/env.h b/ui/aura/env.h
index 31f73666f586b357930a120a79e7f182e016eddb..303e7b66d6de21fb429140a7fd3cf0982bebc0ce 100644
--- a/ui/aura/env.h
+++ b/ui/aura/env.h
@@ -114,6 +114,7 @@ class AURA_EXPORT Env : public ui::EventTarget,
 
  private:
   friend class test::EnvTestHelper;
+  friend class EventInjector;
   friend class MusMouseLocationUpdater;
   friend class Window;
   friend class WindowTreeHost;