ui: Introduce CanvasPainter, remove PaintContext(gfx::Canvas*).

ui/compositor/canvas_painter.h

Index: ui/compositor/canvas_painter.h
diff --git a/ui/compositor/canvas_painter.h b/ui/compositor/canvas_painter.h
new file mode 100644
index 0000000000000000000000000000000000000000..6d8d39023535f1a8f030cd94eeaaa19236f1e9f0
--- /dev/null
+++ b/ui/compositor/canvas_painter.h
@@ -0,0 +1,48 @@
+// 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_CANVAS_PAINTER_H_
+#define UI_COMPOSITOR_CANVAS_PAINTER_H_
+
+#include "base/memory/ref_counted.h"
+#include "ui/compositor/compositor_export.h"
+#include "ui/compositor/paint_context.h"
+#include "ui/gfx/geometry/rect.h"
+
+namespace cc {
+class DisplayItemList;
+}
+
+namespace gfx {
+class Canvas;
+}
+
+namespace ui {
+
+// This class provides a simple helper for painting directly to a bitmap
+// backed canvas (for painting use-cases other than compositing). After
+// constructing an instance of a CanvasPainter, the context() can be used
+// to do painting using the normal composited paint paths. When
+// the painter is destroyed, any painting done with the context() will be
+// rastered into the canvas.
+class COMPOSITOR_EXPORT CanvasPainter {
+ public:
+  CanvasPainter(gfx::Canvas* canvas, float raster_scale_factor);
+  ~CanvasPainter();
+
+  const PaintContext& context() const { return context_; }
+
+ private:
+  gfx::Canvas* const canvas_;
+  const float raster_scale_factor_;
+  const gfx::Rect rect_;
+  scoped_refptr<cc::DisplayItemList> list_;
+  PaintContext context_;
+
+  DISALLOW_COPY_AND_ASSIGN(CanvasPainter);
+};
+
+}  // namespace ui
+
+#endif  // UI_COMPOSITOR_CANVAS_PAINTER_H_