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/client_handler_osr.cc

@@ -146,4 +146,11 @@ void ClientHandlerOsr::OnImeCompositionRangeChanged(
                                               character_bounds);
 }
 
+void ClientHandlerOsr::OnAccessibilityTreeChange(CefRefPtr<CefValue> value) {
+  CEF_REQUIRE_UI_THREAD();
+  if (!osr_delegate_)
+    return;
+  osr_delegate_->UpdateAccessibilityTree(value);
+}
+
 }  // namespace client