Mac: Don't repaint scrollbars every frame

cc/test/fake_scrollbar.h

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_TEST_FAKE_SCROLLBAR_H_
 #define CC_TEST_FAKE_SCROLLBAR_H_
 
 #include "base/compiler_specific.h"
 #include "cc/input/scrollbar.h"
 #include "third_party/skia/include/core/SkColor.h"
 
 namespace cc {
 
 class FakeScrollbar : public Scrollbar {
  public:
   FakeScrollbar();
   FakeScrollbar(bool paint, bool has_thumb, bool is_overlay);
   ~FakeScrollbar() override;
 
   // Scrollbar implementation.
   ScrollbarOrientation Orientation() const override;
   bool IsLeftSideVerticalScrollbar() const override;
   gfx::Point Location() const override;
   bool IsOverlay() const override;
   bool HasThumb() const override;
   int ThumbThickness() const override;
   int ThumbLength() const override;
   gfx::Rect TrackRect() const override;
+  float ThumbOpacity() const override;
+  float TrackOpacity() const override;
+  bool NeedsPaintPart(ScrollbarPart part) const override;
   void PaintPart(SkCanvas* canvas,
                  ScrollbarPart part,
                  const gfx::Rect& content_rect) override;
 
   void set_location(const gfx::Point& location) { location_ = location; }
   void set_track_rect(const gfx::Rect& track_rect) { track_rect_ = track_rect; }
   void set_thumb_thickness(int thumb_thickness) {
       thumb_thickness_ = thumb_thickness;
   }
   void set_thumb_length(int thumb_length) { thumb_length_ = thumb_length; }
   void set_has_thumb(bool has_thumb) { has_thumb_ = has_thumb; }
   SkColor paint_fill_color() const { return SK_ColorBLACK | fill_color_; }
 
+  void set_thumb_opacity(float opacity) { thumb_opacity_ = opacity; }
+  void set_track_opacity(float opacity) { track_opacity_ = opacity; }
+  void set_needs_paint_thumb(bool needs_paint) {
+    needs_paint_thumb_ = needs_paint;
+  }
+  void set_needs_paint_track(bool needs_paint) {
+    needs_paint_track_ = needs_paint;
+  }
+
  private:
   bool paint_;
   bool has_thumb_;
   bool is_overlay_;
   int thumb_thickness_;
   int thumb_length_;
+  float thumb_opacity_;
+  float track_opacity_;
+  bool needs_paint_thumb_;
+  bool needs_paint_track_;
   gfx::Point location_;
   gfx::Rect track_rect_;
   SkColor fill_color_;
 
   DISALLOW_COPY_AND_ASSIGN(FakeScrollbar);
 };
 
 }  // namespace cc
 
 #endif  // CC_TEST_FAKE_SCROLLBAR_H_