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

ui/compositor/paint_cache.cc

+// 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.
+
+#include "ui/compositor/paint_cache.h"
+
+#include "cc/resources/display_item_list.h"
+#include "cc/resources/drawing_display_item.h"
+#include "ui/compositor/paint_context.h"
+
+namespace ui {
+
+PaintCache::PaintCache() {
+}
+
+PaintCache::~PaintCache() {
+}
+
+bool PaintCache::UseCache(const PaintContext& context) {
+  if (!display_item_)
+    return false;
+  DCHECK(context.list_);
+  context.list_->AppendItem(display_item_->Clone());
+  return true;
+}
+
+void PaintCache::SetCache(scoped_ptr<cc::DrawingDisplayItem> item) {
+  display_item_ = item.Pass();
+}
+
+}  // namespace ui