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.

libcef/browser/osr/osr_accessibility_util.h

@@ -0,0 +1,29 @@
+// Copyright 2017 The Chromium Embedded Framework 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_LIBCEF_BROWSER_OSR_ACCESSIBILITY_UTIL_H_
+#define CEF_LIBCEF_BROWSER_OSR_ACCESSIBILITY_UTIL_H_
+#pragma once
+
+#include <vector>
+#include "include/cef_values.h"
+
+namespace content {
+struct AXEventNotificationDetails;
+struct AXLocationChangeNotificationDetails;
+}
+
+namespace osr_accessibility_util {
+
+// Convert Accessibility Event and location updates to CefValue, which may be
+// consumed or serialized with CefJSONWrite.
+CefRefPtr<CefValue> ParseAccessibilityEventData(
+    const std::vector<content::AXEventNotificationDetails>& data);
+
+CefRefPtr<CefValue> ParseAccessibilityLocationData(
+    const std::vector<content::AXLocationChangeNotificationDetails>& data);
+
+}  // namespace osr_accessibility_util
+
+#endif  // CEF_LIBCEF_BROWSER_ACCESSIBILITY_UTIL_H_