ui: Remove a bunch of uses of gfx::Canvas::ExtractImageRep()

ash/common/frame/caption_buttons/frame_caption_button.h

Index: ash/common/frame/caption_buttons/frame_caption_button.h
diff --git a/ash/common/frame/caption_buttons/frame_caption_button.h b/ash/common/frame/caption_buttons/frame_caption_button.h
index 8b6df285b534eeb44c82a17b119ee615cb0d75eb..7437278a0f2692305d46ed103efe8e128689456b 100644
--- a/ash/common/frame/caption_buttons/frame_caption_button.h
+++ b/ash/common/frame/caption_buttons/frame_caption_button.h
@@ -66,10 +66,9 @@ class ASH_EXPORT FrameCaptionButton : public views::CustomButton {
   void OnGestureEvent(ui::GestureEvent* event) override;
 
  private:
-  // Paints |to_center| centered within the button with |alpha|.
-  void PaintCentered(gfx::Canvas* canvas,
-                     const gfx::ImageSkia& to_center,
-                     int alpha);
+  // Determines what alpha to use for the icon based on animation and
+  // active state.
+  int GetAlphaForIcon(int base_alpha) const;
 
   // The button's current icon.
   CaptionButtonIcon icon_;