Moves WorkspaceEventHandler to ash/common

ash/common/wm_shell.h

Index: ash/common/wm_shell.h
diff --git a/ash/common/wm_shell.h b/ash/common/wm_shell.h
index 3db0a06101b8f4178ba92b0bbf246f701d6bb07e..ba690c1462138afba315009c86cc17876d8b26af 100644
--- a/ash/common/wm_shell.h
+++ b/ash/common/wm_shell.h
@@ -64,6 +64,7 @@ class WindowSelectorController;
 class WmActivationObserver;
 class WmDisplayObserver;
 class WmWindow;
+class WorkspaceEventHandler;
 
 enum class TaskSwitchSource;
 
@@ -270,6 +271,9 @@ class ASH_EXPORT WmShell {
   virtual std::unique_ptr<wm::MaximizeModeEventHandler>
   CreateMaximizeModeEventHandler() = 0;
 
+  virtual std::unique_ptr<WorkspaceEventHandler> CreateWorkspaceEventHandler(
+      WmWindow* workspace_window) = 0;
+
   virtual std::unique_ptr<ScopedDisableInternalMouseAndKeyboard>
   CreateScopedDisableInternalMouseAndKeyboard() = 0;