ui: Cache the output of View::OnPaint when the View isn't invalid.

ui/compositor/paint_cache.h

+// Copyright 2015 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 UI_COMPOSITOR_PAINT_CACHE_H_
+#define UI_COMPOSITOR_PAINT_CACHE_H_
+
+#include "base/memory/scoped_ptr.h"
+#include "ui/compositor/compositor_export.h"
+
+namespace cc {
+class DrawingDisplayItem;
+}
+
+namespace gfx {
+class Rect;

[sky, 2015/04/23 16:29:00]

nit: you don't use rect here (or need the rect include in the .cc).

+}
+
+namespace ui {
+class PaintContext;
+class PaintRecorder;
+
+// A class that holds the output of a PaintRecorder to be reused when the
+// object that created the PaintRecorder has not been changed/invalidated.
+class COMPOSITOR_EXPORT PaintCache {
+ public:
+  PaintCache();
+  ~PaintCache();
+
+  bool UseCache(const PaintContext& context);

[sky, 2015/04/23 16:29:00]

Document what this does and what the return value means.

+
+ private:
+  // Only PaintRecorder can modify these.
+  friend PaintRecorder;
+
+  void SetCache(scoped_ptr<cc::DrawingDisplayItem> item);
+
+  scoped_ptr<cc::DrawingDisplayItem> display_item_;
+
+  DISALLOW_COPY_AND_ASSIGN(PaintCache);
+};
+
+}  // namespace ui
+
+#endif  // UI_COMPOSITOR_PAINT_CACHE_H_