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/cpptoc/accessibility_handler_cpptoc.cc

@@ -0,0 +1,81 @@
+// 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/accessibility_handler_cpptoc.h"
+#include "libcef_dll/ctocpp/value_ctocpp.h"
+
+
+namespace {
+
+// MEMBER FUNCTIONS - Body may be edited by hand.
+
+void CEF_CALLBACK accessibility_handler_on_accessibility_tree_change(
+    struct _cef_accessibility_handler_t* self, struct _cef_value_t* value) {
+  // AUTO-GENERATED CONTENT - DELETE THIS COMMENT BEFORE MODIFYING
+
+  DCHECK(self);
+  if (!self)
+    return;
+  // Verify param: value; type: refptr_diff
+  DCHECK(value);
+  if (!value)
+    return;
+
+  // Execute
+  CefAccessibilityHandlerCppToC::Get(self)->OnAccessibilityTreeChange(
+      CefValueCToCpp::Wrap(value));
+}
+
+void CEF_CALLBACK accessibility_handler_on_accessibility_location_change(
+    struct _cef_accessibility_handler_t* self, struct _cef_value_t* value) {
+  // AUTO-GENERATED CONTENT - DELETE THIS COMMENT BEFORE MODIFYING
+
+  DCHECK(self);
+  if (!self)
+    return;
+  // Verify param: value; type: refptr_diff
+  DCHECK(value);
+  if (!value)
+    return;
+
+  // Execute
+  CefAccessibilityHandlerCppToC::Get(self)->OnAccessibilityLocationChange(
+      CefValueCToCpp::Wrap(value));
+}
+
+}  // namespace
+
+
+// CONSTRUCTOR - Do not edit by hand.
+
+CefAccessibilityHandlerCppToC::CefAccessibilityHandlerCppToC() {
+  GetStruct()->on_accessibility_tree_change =
+      accessibility_handler_on_accessibility_tree_change;
+  GetStruct()->on_accessibility_location_change =
+      accessibility_handler_on_accessibility_location_change;
+}
+
+template<> CefRefPtr<CefAccessibilityHandler> CefCppToCRefCounted<CefAccessibilityHandlerCppToC,
+    CefAccessibilityHandler, cef_accessibility_handler_t>::UnwrapDerived(
+    CefWrapperType type, cef_accessibility_handler_t* s) {
+  NOTREACHED() << "Unexpected class type: " << type;
+  return NULL;
+}
+
+#if DCHECK_IS_ON()
+template<> base::AtomicRefCount CefCppToCRefCounted<CefAccessibilityHandlerCppToC,
+    CefAccessibilityHandler, cef_accessibility_handler_t>::DebugObjCt = 0;
+#endif
+
+template<> CefWrapperType CefCppToCRefCounted<CefAccessibilityHandlerCppToC,
+    CefAccessibilityHandler, cef_accessibility_handler_t>::kWrapperType =
+    WT_ACCESSIBILITY_HANDLER;