Moves WorkspaceEventHandler to ash/common

ash/common/wm_window.h

Index: ash/common/wm_window.h
diff --git a/ash/common/wm_window.h b/ash/common/wm_window.h
index 5488e0320028b93f631a391ef1a7ee433efa3c20..6dfd2cad7ef6a110c7bf055cadf9382b651b3dff 100644
--- a/ash/common/wm_window.h
+++ b/ash/common/wm_window.h
@@ -309,6 +309,8 @@ class ASH_EXPORT WmWindow {
   // this window handles the events itself; the handler does not recieve events
   // from embedded windows. This only supports windows with internal widgets;
   // see GetInternalWidget(). Ownership of the handler is not transferred.
+  //
+  // Also note that the target of these events is always an aura::Window.
   virtual void AddLimitedPreTargetHandler(ui::EventHandler* handler) = 0;
   virtual void RemoveLimitedPreTargetHandler(ui::EventHandler* handler) = 0;