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/browser/browser_platform_delegate.cc

@@ -202,6 +202,16 @@ void CefBrowserPlatformDelegate::DragSourceSystemDragEnded() {
   NOTREACHED();
 }
 
+void CefBrowserPlatformDelegate::AccessibilityEventReceived(
+    const std::vector<content::AXEventNotificationDetails>& eventData) {
+  NOTREACHED();
+}
+
+void CefBrowserPlatformDelegate::AccessibilityLocationChangesReceived(
+    const std::vector<content::AXLocationChangeNotificationDetails>& locData) {
+  NOTREACHED();
+}
+
 // static
 int CefBrowserPlatformDelegate::TranslateModifiers(uint32 cef_modifiers) {
   int webkit_modifiers = 0;