Add EventRewriterController as an aura::Env observer.

chrome/browser/chromeos/events/event_rewriter_controller.cc

Index: chrome/browser/chromeos/events/event_rewriter_controller.cc
diff --git a/chrome/browser/chromeos/events/event_rewriter_controller.cc b/chrome/browser/chromeos/events/event_rewriter_controller.cc
index 8b3cc5b40c8342b16b5f3756788f52cafe8c83ae..62fe12a4197b050e4138f68ab931130a66b2559d 100644
--- a/chrome/browser/chromeos/events/event_rewriter_controller.cc
+++ b/chrome/browser/chromeos/events/event_rewriter_controller.cc
@@ -5,15 +5,19 @@
 #include "chrome/browser/chromeos/events/event_rewriter_controller.h"
 
 #include "ash/shell.h"
+#include "ui/aura/env.h"
 #include "ui/aura/window_tree_host.h"
 #include "ui/events/event_source.h"
 
 namespace chromeos {
 
 EventRewriterController::EventRewriterController() : initialized_(false) {
+  // Add the controller as an observer for new root windows.
+  aura::Env::GetInstance()->AddObserver(this);
 }
 
 EventRewriterController::~EventRewriterController() {
+  aura::Env::GetInstance()->RemoveObserver(this);
   // Remove the rewriters from every root window EventSource and destroy them.
   for (EventRewriters::iterator rewriter_iter = rewriters_.begin();
        rewriter_iter != rewriters_.end();