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_node.h

@@ -0,0 +1,116 @@
+// 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_NODE_H_
+#define CEF_TESTS_CEFCLIENT_BROWSER_OSR_ACCESSIBILITY_NODE_H_
+#pragma once
+
+#include <vector>
+
+#include "include/cef_browser.h"
+
+#if defined(OS_MACOSX)
+#ifdef __OBJC__
+@class NSObject;
+#else
+class NSObject;
+#endif
+typedef NSObject CefNativeAccessible;
+#elif defined(OS_WIN)
+struct IAccessible;
+typedef IAccessible CefNativeAccessible;
+#else
+#error "Unsupported platform"
+#endif
+
+namespace client {
+
+class OsrAccessibilityHelper;
+
+// OsrAXNode is the base class for implementation for the NSAccessibility
+// protocol for interacting with VoiceOver and other accessibility clients.
+class OsrAXNode {
+ public:
+  // Create and return the platform specific OsrAXNode Object.
+  static OsrAXNode* CreateNode(CefRefPtr<CefDictionaryValue> value,
+                               OsrAccessibilityHelper* helper);
+
+  // Update Value.
+  void UpdateValue(CefRefPtr<CefDictionaryValue> value);
+
+  // Fire a platform-specific notification that an event has occurred on
+  // this object.
+  void NotifyAccessibilityEvent(std::string event_type) const;
+
+  // Call Destroy rather than deleting this, because the subclass may
+  // use reference counting.
+  void Destroy();
+
+  // Return NSAccessibility Object for Mac/ IAccessible for Windows
+  CefNativeAccessible* GetNativeAccessibleObject(OsrAXNode* parent);
+
+  CefNativeAccessible* GetParentAccessibleObject() const {
+    return parent_? parent_->platform_accessibility_ : NULL;
+  }
+
+  OsrAccessibilityHelper* GetAccessibilityHelper() const {
+    return accessibility_helper_;
+  };
+
+  int GetChildCount() const {
+    return static_cast<int>(child_ids_.size());
+  }
+
+  // Return the Child at the specified index
+  OsrAXNode* ChildAtIndex(int index) const;
+
+  const CefString& AxRole() const {
+    return role_;
+  }
+
+  int OsrAXNodeId() const {
+    return node_id_;
+  }
+
+  const CefString& AxValue() const {
+    return value_;
+  }
+
+  const CefString& AxName() const {
+    return name_;
+  }
+
+  const CefString& AxDescription() const {
+    return description_;
+  }
+
+  CefRect AxLocation() const;
+
+  CefWindowHandle GetWindowHandle() const;
+
+  CefRefPtr<CefBrowser> GetBrowser() const;
+
+  void SetParent(OsrAXNode* parent);
+
+ protected:
+  OsrAXNode(CefRefPtr<CefDictionaryValue> value,
+            OsrAccessibilityHelper* helper);
+
+  int node_id_;
+  CefString role_;
+  CefString value_;
+  CefString name_;
+  CefString description_;
+  CefRect location_;
+  std::vector<int> child_ids_;
+  CefNativeAccessible* platform_accessibility_;
+  OsrAXNode* parent_;
+  int offset_container_id_;
+  OsrAccessibilityHelper* accessibility_helper_;
+  CefRefPtr<CefDictionaryValue> attributes_;
+};
+
+}  // namespace client
+
+#endif  // CEF_TESTS_CEFCLIENT_BROWSER_OSR_ACCESSIBILITY_NODE_H_