Bug 1847933 - Fix indeterminate radio button rendering on macOS' native theme. r=mac-reviewers,bradwerth

This fixes a regression introduced in:

  https://hg.mozilla.org/mozilla-central/rev/5cc085b16d94f258f011bbacbb52862ba1559465#l1.138

<input type=checkbox> can match :indeterminate but that was getting
rendered as checked incorrectly by the native theme. Reintroduce the
relevant check.

Differential Revision: https://phabricator.services.mozilla.com/D186011

widget/cocoa/nsNativeThemeCocoa.mm

@@ -2221,11 +2221,11 @@ Maybe<nsNativeThemeCocoa::WidgetInfo> nsNativeThemeCocoa::ComputeWidgetInfo(
 
     case StyleAppearance::Checkbox:
     case StyleAppearance::Radio: {
-      bool isCheckbox = (aAppearance == StyleAppearance::Checkbox);
+      bool isCheckbox = aAppearance == StyleAppearance::Checkbox;
 
       CheckboxOrRadioParams params;
       params.state = CheckboxOrRadioState::eOff;
-      if (elementState.HasState(ElementState::INDETERMINATE)) {
+      if (isCheckbox && elementState.HasState(ElementState::INDETERMINATE)) {
         params.state = CheckboxOrRadioState::eIndeterminate;
       } else if (elementState.HasState(ElementState::CHECKED)) {
         params.state = CheckboxOrRadioState::eOn;