mash: Avoid KeyboardDrivenEventRewriter::GetInstance crash.

chrome/browser/chromeos/chrome_browser_main_chromeos.cc

Index: chrome/browser/chromeos/chrome_browser_main_chromeos.cc
diff --git a/chrome/browser/chromeos/chrome_browser_main_chromeos.cc b/chrome/browser/chromeos/chrome_browser_main_chromeos.cc
index 1f14e388d26e590b75697fcb9c8e349c434f133e..7adb0635abcbb156ae305a93598300868889d35f 100644
--- a/chrome/browser/chromeos/chrome_browser_main_chromeos.cc
+++ b/chrome/browser/chromeos/chrome_browser_main_chromeos.cc
@@ -741,6 +741,7 @@ void ChromeBrowserMainPartsChromeos::PostBrowserStart() {
     login_lock_state_notifier_.reset(new LoginLockStateNotifier);
     data_promo_notification_.reset(new DataPromoNotification());
 
+    // TODO(mash): Support EventRewriterController; see crbug.com/647781
     keyboard_event_rewriters_.reset(new EventRewriterController());
     keyboard_event_rewriters_->AddEventRewriter(
         std::unique_ptr<ui::EventRewriter>(new KeyboardDrivenEventRewriter()));