events: Move the PlatformEventSource creation in aura::Env.

ui/aura/env.h

Index: ui/aura/env.h
diff --git a/ui/aura/env.h b/ui/aura/env.h
index c188178b7d60c5253f065073fa4f369d8fc557bf..1d6c19de3fb07a4a15b1cab5fd84e714daa09438 100644
--- a/ui/aura/env.h
+++ b/ui/aura/env.h
@@ -12,6 +12,9 @@
 #include "ui/events/event_target.h"
 #include "ui/gfx/point.h"
 
+namespace ui {
+class PlatformEventSource;
+}
 namespace aura {
 
 namespace test {
@@ -86,6 +89,7 @@ class AURA_EXPORT Env : public ui::EventTarget {
   bool is_touch_down_;
 
   scoped_ptr<InputStateLookup> input_state_lookup_;
+  scoped_ptr<ui::PlatformEventSource> event_source_;
 
   DISALLOW_COPY_AND_ASSIGN(Env);
 };