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

ui/aura/env.cc

Index: ui/aura/env.cc
diff --git a/ui/aura/env.cc b/ui/aura/env.cc
index 61b7f27aa28a10e0961826767ec9b02790785fa6..f701695738958abcfa6dd6e4753e31cf068fc7be 100644
--- a/ui/aura/env.cc
+++ b/ui/aura/env.cc
@@ -8,6 +8,7 @@
 #include "ui/aura/input_state_lookup.h"
 #include "ui/compositor/compositor.h"
 #include "ui/events/event_target_iterator.h"
+#include "ui/events/platform/platform_event_source.h"
 
 namespace aura {
 
@@ -68,6 +69,9 @@ bool Env::IsMouseButtonDown() const {
 
 void Env::Init() {
   ui::Compositor::Initialize();
+
+  if (!ui::PlatformEventSource::GetInstance())
+    event_source_ = ui::PlatformEventSource::CreateDefault();
 }
 
 void Env::NotifyWindowInitialized(Window* window) {