Implement hit tests/client area.

ui/aura/mus/window_tree_host_mus_delegate.h

Index: ui/aura/mus/window_tree_host_mus_delegate.h
diff --git a/ui/aura/mus/window_tree_host_mus_delegate.h b/ui/aura/mus/window_tree_host_mus_delegate.h
index e93dcfcaa912ba3117bd7630a914cf937d8b0bea..7c85290335403f0949df746a42ba3df4473e4d8b 100644
--- a/ui/aura/mus/window_tree_host_mus_delegate.h
+++ b/ui/aura/mus/window_tree_host_mus_delegate.h
@@ -25,6 +25,16 @@ class AURA_EXPORT WindowTreeHostMusDelegate {
       WindowTreeHostMus* window_tree_host,
       const gfx::Rect& bounds) = 0;
 
+  // Called when the client area of a WindowTreeHostMus is about to change.
+  virtual void OnWindowTreeHostClientAreaWillChange(
+      WindowTreeHostMus* window_tree_host,
+      const gfx::Insets& client_area) = 0;
+
+  // Called when the hit test mask is about to be cleared.
+  virtual void OnWindowTreeHostHitTestMaskWillChange(
+      WindowTreeHostMus* window_tree_host,
+      const base::Optional<gfx::Rect>& mask_rect) = 0;
+
   // Called when a WindowTreeHostMus is created without a WindowPort.
   virtual std::unique_ptr<WindowPortMus> CreateWindowPortForTopLevel() = 0;