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/root_window_win.cc

@@ -541,6 +541,18 @@ LRESULT CALLBACK RootWindowWin::RootWndProc(HWND hWnd, UINT message,
         return 0;
       break;
 
+    case WM_GETOBJECT: {
+      // Only the lower 32 bits of lParam are valid when checking the object id
+      // because it sometimes gets sign-extended incorrectly (but not always).
+      DWORD obj_id = static_cast<DWORD>(static_cast<DWORD_PTR>(lParam));
+
+      // Accessibility readers will send an OBJID_CLIENT message.
+      if (static_cast<DWORD>(OBJID_CLIENT) == obj_id) {
+        if (self->GetBrowser() && self->GetBrowser()->GetHost())
+          self->GetBrowser()->GetHost()->SetAccessibilityState(STATE_ENABLED);
+      }
+    } break;
+
     case WM_PAINT:
       self->OnPaint();
       return 0;