cc: Detach spiral iterator implementation to separate file.

cc/base/reverse_spiral_iterator.h

Index: cc/base/reverse_spiral_iterator.h
diff --git a/cc/base/reverse_spiral_iterator.h b/cc/base/reverse_spiral_iterator.h
new file mode 100644
index 0000000000000000000000000000000000000000..d6c5d9e1d859fb9503ef6d5ef7752c8e5ba3fbbd
--- /dev/null
+++ b/cc/base/reverse_spiral_iterator.h
@@ -0,0 +1,71 @@
+// Copyright 2016 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_BASE_REVERSE_SPIRAL_ITERATOR_H_
+#define CC_BASE_REVERSE_SPIRAL_ITERATOR_H_
+
+#include "base/logging.h"
+#include "cc/base/cc_export.h"
+#include "cc/base/index_rect.h"
+
+namespace cc {
+
+// The spiral iterator which iterates in reverse direction based on directions
+// around the center rect in the given region. If the center rect is at index
+// (2, 2), reverse spiral iterator gives following sequence on iterating.
+//
+//    x 0   1   2   3   4
+//  y ┌───┬───┬───┬───┬───┐
+//  0 │  9│ 10│ 11│ 12│ 13│
+//    ├───┼───┼───┼───┼───┤
+//  1 │  8│ 21│ 22│ 23│ 14│
+//    ├───┼───┼───┼───┼───┤
+//  2 │  7│ 20│  *│ 24│ 15│
+//    ├───┼───┼───┼───┼───┤
+//  3 │  6│ 19│ 18│ 17│ 16│
+//    ├───┼───┼───┼───┼───┤
+//  4 │  5│  4│  3│  2│  1│
+//    └───┴───┴───┴───┴───┘
+class CC_EXPORT ReverseSpiralIterator {
+ public:
+  ReverseSpiralIterator();
+  ReverseSpiralIterator(const IndexRect& around_index_rect,
+                        const IndexRect& consider_index_rect,
+                        const IndexRect& ignore_index_rect);
+
+  ~ReverseSpiralIterator() = default;
+
+  operator bool() const;
+  ReverseSpiralIterator& operator++();
+  int index_x() const { return index_x_; }
+  int index_y() const { return index_y_; }
+
+ private:
+  int current_step_count() const {
+    return (direction_ == UP || direction_ == DOWN) ? vertical_step_count_
+                                                    : horizontal_step_count_;
+  }
+
+  bool needs_direction_switch() const;
+  void switch_direction();
+
+  enum Direction { LEFT, UP, RIGHT, DOWN };
+
+  IndexRect around_index_rect_;
+  IndexRect consider_index_rect_;
+  IndexRect ignore_index_rect_;
+  int index_x_;
+  int index_y_;
+
+  Direction direction_;
+  int delta_x_;
+  int delta_y_;
+  int current_step_;
+  int horizontal_step_count_;
+  int vertical_step_count_;
+};
+
+}  // namespace cc
+
+#endif  // CC_BASE_REVERSE_SPIRAL_ITERATOR_H_