Implement accessibility enhancements (issue #1217)

- Add new CefBrowserHost::SetAccessibilityState method for toggling
  accessibility state when readers are detected by the client.
- Add new CefAccessibilityHandler interface for the delivery of
  accessibility notifications to windowless (OSR) clients.
- Fix delivery of CefFocusHandler callbacks to windowless clients.
- cefclient: Add example windowless accessibility implementation on Windows and macOS.
- cefclient: Automatically detect screen readers on Windows and macOS.

tests/cefclient/browser/osr_accessibility_helper.h

@@ -0,0 +1,63 @@
+// Copyright 2017 The Chromium Embedded Framework Authors. Portions copyright
+// 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 CEF_TESTS_CEFCLIENT_BROWSER_OSR_ACCESSIBILITY_HELPER_H_
+#define CEF_TESTS_CEFCLIENT_BROWSER_OSR_ACCESSIBILITY_HELPER_H_
+
+#include <map>
+
+#include "include/cef_browser.h"
+
+namespace client {
+
+class OsrAXNode;
+
+// Helper class that abstracts Renderer Accessibility tree and provides a
+// uniform interface to be consumed by IAccessible interface on Windows and
+// NSAccessibility implementation on Mac in CefClient.
+class OsrAccessibilityHelper {
+ public:
+  OsrAccessibilityHelper(CefRefPtr<CefValue> value,
+                         CefRefPtr<CefBrowser> browser);
+
+  void UpdateAccessibilityTree(CefRefPtr<CefValue> value);
+
+  OsrAXNode* GetRootNode() const {
+    return GetNode(root_node_id_);
+  }
+
+  OsrAXNode* GetFocusedNode() const {
+    return GetNode(focused_node_id_);
+  }
+
+  CefWindowHandle GetWindowHandle() const {
+    return browser_->GetHost()->GetWindowHandle();
+  }
+
+  CefRefPtr<CefBrowser> GetBrowser() const {
+    return browser_;
+  };
+
+  OsrAXNode* GetNode(int nodeId) const;
+
+ private:
+  OsrAXNode* CreateNode(OsrAXNode* parent, CefRefPtr<CefDictionaryValue> value);
+
+  void Reset();
+
+  void UpdateLayout(CefRefPtr<CefDictionaryValue> update);
+
+  void UpdateFocusedNode(CefRefPtr<CefDictionaryValue> update);
+
+  // Destroy the node and remove from Map
+  void DestroyNode(OsrAXNode* node);
+  int root_node_id_;
+  int focused_node_id_;
+  CefRefPtr<CefBrowser> browser_;
+  std::map<int, OsrAXNode*> accessibility_node_map_;
+};
+
+}  // namespace client
+
+#endif  // CEF_TESTS_CEFCLIENT_BROWSER_OSR_ACCESSIBILITY_HELPER_H_