chromeos: makes MoveEventHandler --mash specific

It's not applicable to ws2 (because the pre-target handlers installed by
ash take care of it).

BUG=842295
TEST=none

Change-Id: I57ae1c684580233233bff13186560c2790f56c79
Reviewed-on: https://chromium-review.googlesource.com/1088128Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564888}

ash/wm/move_event_handler.cc

@@ -116,7 +116,6 @@ WorkspaceEventHandlerMash* MoveEventHandler::GetWorkspaceEventHandlerMash() {
 
 void MoveEventHandler::OnMouseEvent(ui::MouseEvent* event) {
   toplevel_window_event_handler_.OnMouseEvent(event, window_);
-  // TODO(crbug.com/842295): Add support for window-service as a library.
   if (!toplevel_window_event_handler_.is_drag_in_progress() &&
       window_manager_client_ &&
       (event->type() == ui::ET_POINTER_MOVED ||

ash/wm/non_client_frame_controller.cc

@@ -191,8 +191,10 @@ class WmNativeWidgetAura : public views::NativeWidgetAura {
 
   // views::NativeWidgetAura:
   views::NonClientFrameView* CreateNonClientFrameView() override {
-    move_event_handler_ = std::make_unique<MoveEventHandler>(
-        window_manager_client_, GetNativeView());
+    if (window_manager_client_) {
+      move_event_handler_ = std::make_unique<MoveEventHandler>(
+          window_manager_client_, GetNativeView());
+    }
     // TODO(sky): investigate why we have this. Seems this should be the same
     // as not specifying client area insets.
     if (remove_standard_frame_)
@@ -225,6 +227,8 @@ class WmNativeWidgetAura : public views::NativeWidgetAura {
   const bool enable_immersive_;
   const mojom::WindowStyle window_style_;
 
+  // TODO: this is no longer necessary once --mash is removed,
+  // https://crbug.com/842365.
   std::unique_ptr<MoveEventHandler> move_event_handler_;
 
   aura::WindowManagerClient* window_manager_client_;