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_dll/ctocpp/accessibility_handler_ctocpp.cc

@@ -0,0 +1,75 @@
+// Copyright (c) 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.
+//
+// ---------------------------------------------------------------------------
+//
+// This file was generated by the CEF translator tool. If making changes by
+// hand only do so within the body of existing method and function
+// implementations. See the translator.README.txt file in the tools directory
+// for more information.
+//
+
+#include "libcef_dll/cpptoc/value_cpptoc.h"
+#include "libcef_dll/ctocpp/accessibility_handler_ctocpp.h"
+
+
+// VIRTUAL METHODS - Body may be edited by hand.
+
+void CefAccessibilityHandlerCToCpp::OnAccessibilityTreeChange(
+    CefRefPtr<CefValue> value) {
+  cef_accessibility_handler_t* _struct = GetStruct();
+  if (CEF_MEMBER_MISSING(_struct, on_accessibility_tree_change))
+    return;
+
+  // AUTO-GENERATED CONTENT - DELETE THIS COMMENT BEFORE MODIFYING
+
+  // Verify param: value; type: refptr_diff
+  DCHECK(value.get());
+  if (!value.get())
+    return;
+
+  // Execute
+  _struct->on_accessibility_tree_change(_struct,
+      CefValueCppToC::Wrap(value));
+}
+
+void CefAccessibilityHandlerCToCpp::OnAccessibilityLocationChange(
+    CefRefPtr<CefValue> value) {
+  cef_accessibility_handler_t* _struct = GetStruct();
+  if (CEF_MEMBER_MISSING(_struct, on_accessibility_location_change))
+    return;
+
+  // AUTO-GENERATED CONTENT - DELETE THIS COMMENT BEFORE MODIFYING
+
+  // Verify param: value; type: refptr_diff
+  DCHECK(value.get());
+  if (!value.get())
+    return;
+
+  // Execute
+  _struct->on_accessibility_location_change(_struct,
+      CefValueCppToC::Wrap(value));
+}
+
+
+// CONSTRUCTOR - Do not edit by hand.
+
+CefAccessibilityHandlerCToCpp::CefAccessibilityHandlerCToCpp() {
+}
+
+template<> cef_accessibility_handler_t* CefCToCppRefCounted<CefAccessibilityHandlerCToCpp,
+    CefAccessibilityHandler, cef_accessibility_handler_t>::UnwrapDerived(
+    CefWrapperType type, CefAccessibilityHandler* c) {
+  NOTREACHED() << "Unexpected class type: " << type;
+  return NULL;
+}
+
+#if DCHECK_IS_ON()
+template<> base::AtomicRefCount CefCToCppRefCounted<CefAccessibilityHandlerCToCpp,
+    CefAccessibilityHandler, cef_accessibility_handler_t>::DebugObjCt = 0;
+#endif
+
+template<> CefWrapperType CefCToCppRefCounted<CefAccessibilityHandlerCToCpp,
+    CefAccessibilityHandler, cef_accessibility_handler_t>::kWrapperType =
+    WT_ACCESSIBILITY_HANDLER;