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.

include/cef_accessibility_handler.h

@@ -0,0 +1,66 @@
+// Copyright (c) 2017 Marshall A. Greenblatt. All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//    * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//    * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//    * Neither the name of Google Inc. nor the name Chromium Embedded
+// Framework nor the names of its contributors may be used to endorse
+// or promote products derived from this software without specific prior
+// written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+//
+// ---------------------------------------------------------------------------
+//
+// The contents of this file must follow a specific format in order to
+// support the CEF translator tool. See the translator.README.txt file in the
+// tools directory for more information.
+//
+
+#ifndef CEF_INCLUDE_CEF_ACCESSIBILITY_HANDLER_H_
+#define CEF_INCLUDE_CEF_ACCESSIBILITY_HANDLER_H_
+#pragma once
+
+#include "include/cef_values.h"
+
+///
+// Implement this interface to receive accessibility notification when
+// accessibility events have been registered. The methods of this class will
+// be called on the UI thread.
+///
+/*--cef(source=client)--*/
+class CefAccessibilityHandler : public virtual CefBaseRefCounted {
+ public:
+  ///
+  // Called after renderer process sends accessibility tree changes to the
+  // browser process.
+  ///
+  /*--cef()--*/
+  virtual void OnAccessibilityTreeChange(CefRefPtr<CefValue> value) =0;
+
+  ///
+  // Called after renderer process sends accessibility location changes to the
+  // browser process.
+  ///
+  /*--cef()--*/
+  virtual void OnAccessibilityLocationChange(CefRefPtr<CefValue> value) =0;
+};
+
+#endif  // CEF_INCLUDE_CEF_ACCESSIBILITY_HANDLER_H_