WIP - mask a view's layer by its view coordinates

ui/views/controls/button/md_text_button.h

Index: ui/views/controls/button/md_text_button.h
diff --git a/ui/views/controls/button/md_text_button.h b/ui/views/controls/button/md_text_button.h
index b691b0f0d2dc600fd32d68d629a4cae32c593bd6..83db0d6e86621c21fe1c5c6558ed7f243f31303b 100644
--- a/ui/views/controls/button/md_text_button.h
+++ b/ui/views/controls/button/md_text_button.h
@@ -36,6 +36,13 @@ class VIEWS_EXPORT MdTextButton : public LabelButton {
   void OnPaintBackground(gfx::Canvas* canvas) override;
 
   // LabelButton:
+/*
+void OnPaint(gfx::Canvas* canvas) override;
+void PaintChildren(const ui::PaintContext& context);
+*/
+  bool GetNeedsNotificationWhenVisibleBoundsChange() const override;
+  void OnVisibleBoundsChanged() override;
+  void OnLayerBoundsChanged(const gfx::Rect& old_bounds) override;
   void OnFocus() override;
   void OnBlur() override;
   void OnNativeThemeChanged(const ui::NativeTheme* theme) override;
@@ -63,6 +70,10 @@ class VIEWS_EXPORT MdTextButton : public LabelButton {
   // True if this button uses prominent styling (blue fill, etc.).
   bool is_prominent_;
 
+  gfx::Rect last_visible_bounds_;
+  std::unique_ptr<ui::LayerDelegate> mask_layer_delegate_;
+  ui::Layer mask_layer_;
+
   // When set, this provides the background color.
   base::Optional<SkColor> bg_color_override_;