Fix sticky keys crash when handling synthetic events without a native event.

ash/sticky_keys/sticky_keys_controller.h

Index: ash/sticky_keys/sticky_keys_controller.h
diff --git a/ash/sticky_keys/sticky_keys_controller.h b/ash/sticky_keys/sticky_keys_controller.h
index 792f2c58b0b32dbba3dc133f0ab8ee987ae14400..c9c158015fc0003079793085afc04058bfc74e62 100644
--- a/ash/sticky_keys/sticky_keys_controller.h
+++ b/ash/sticky_keys/sticky_keys_controller.h
@@ -147,15 +147,18 @@ class ASH_EXPORT StickyKeysHandler {
     StickyKeysHandlerDelegate();
     virtual ~StickyKeysHandlerDelegate();
 
-    // Dispatches keyboard event synchronously.
+    // Dispatches keyboard event synchronously. |event| is an event that has
+    // been previously dispatched.
     virtual void DispatchKeyEvent(ui::KeyEvent* event,
                                   aura::Window* target) = 0;
 
-    // Dispatches mouse event synchronously.
+    // Dispatches mouse event synchronously. |event| is an event that has
+    // been previously dispatched.
     virtual void DispatchMouseEvent(ui::MouseEvent* event,
                                     aura::Window* target) = 0;
 
-    // Dispatches scroll event synchronously.
+    // Dispatches scroll event synchronously. |event| is an event that has
+    // been previously dispatched.
     virtual void DispatchScrollEvent(ui::ScrollEvent* event,
                                      aura::Window* target) = 0;
   };