First step to move common accessibility code out of content.

ui/accessibility/ax_serializable_tree.h

Index: ui/accessibility/ax_serializable_tree.h
diff --git a/ui/accessibility/ax_serializable_tree.h b/ui/accessibility/ax_serializable_tree.h
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+++ b/ui/accessibility/ax_serializable_tree.h
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+// Copyright (c) 2013 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_ACCESSIBILITY_AX_SERIALIZABLE_TREE_H_
+#define UI_ACCESSIBILITY_AX_SERIALIZABLE_TREE_H_
+
+#include "ui/accessibility/ax_tree.h"
+#include "ui/accessibility/ax_tree_source.h"
+
+namespace ui {
+
+class AX_EXPORT AXSerializableTree : public AXTree {
+ public:
+  AXSerializableTree();
+  explicit AXSerializableTree(const AXTreeUpdate& initial_state);
+  virtual ~AXSerializableTree();
+
+  // Create a TreeSource adapter for this tree. The client gets ownership
+  // of the return value and should delete it when done.
+  virtual AXTreeSource<AXNode>* CreateTreeSource();
+};
+
+}  // namespace ui
+
+#endif  // UI_ACCESSIBILITY_AX_TREE_H_