Aura-Mus: Allocate a LocalSurfaceId on size change

ui/aura/test/mus/test_window_tree.h

Index: ui/aura/test/mus/test_window_tree.h
diff --git a/ui/aura/test/mus/test_window_tree.h b/ui/aura/test/mus/test_window_tree.h
index 59cc06939d782f0b8387afcd4766e6b3bb8b2f34..916e8a1d861676edd9397ed3d0693b7a06b1dab5 100644
--- a/ui/aura/test/mus/test_window_tree.h
+++ b/ui/aura/test/mus/test_window_tree.h
@@ -94,6 +94,10 @@ class TestWindowTree : public ui::mojom::WindowTree {
     return last_hit_test_mask_;
   }
 
+  const base::Optional<cc::LocalSurfaceId>& last_local_surface_id() const {
+    return last_local_surface_id_;
+  }
+
  private:
   struct Change {
     WindowTreeChangeType type;
@@ -228,6 +232,8 @@ class TestWindowTree : public ui::mojom::WindowTree {
 
   base::Optional<gfx::Rect> last_hit_test_mask_;
 
+  base::Optional<cc::LocalSurfaceId> last_local_surface_id_;
+
   DISALLOW_COPY_AND_ASSIGN(TestWindowTree);
 };