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/osr/browser_platform_delegate_osr.h

@@ -88,6 +88,12 @@ class CefBrowserPlatformDelegateOsr :
   void DragSourceEndedAt(int x, int y,
                          cef_drag_operations_mask_t op) override;
   void DragSourceSystemDragEnded() override;
+  void AccessibilityEventReceived(
+      const std::vector<content::AXEventNotificationDetails>& eventData)
+      override;
+  void AccessibilityLocationChangesReceived(
+      const std::vector<content::AXLocationChangeNotificationDetails>& locData)
+      override;
 
   // CefBrowserPlatformDelegateNative::WindowlessHandler methods:
   CefWindowHandle GetParentWindowHandle() const override;