webView.scrollView.indicatorStyle does not work on iOS 15

[Daohan Chong]

Created attachment 442467 [details]
The demo project (please run it using iOS 14 and 15 simulators respectively)

Previously, on iOS 14, this kind of code can work perfectly:

```swift
  override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
    super.traitCollectionDidChange(previousTraitCollection)

    switch traitCollection.userInterfaceStyle {
    case .light:
      webView.scrollView.indicatorStyle = .black
    case .dark:
      webView.scrollView.indicatorStyle = .white
    default:
      break
    }
  }
```

However, after iOS 15, it stopped from working. We cannot find any related documentation about customizing the color of the scroll indicator.

[Simon Fraser (smfr)]

<rdar://82816423>

[Devin Rousso]

Created attachment 443361 [details]
Patch

[EWS]

Committed r285424 (243980@main): <https://commits.webkit.org/243980@main>

All reviewed patches have been landed. Closing bug and clearing flags on attachment 443361 [details].

[Florent Morin]

This fix don't work on iOS when view is not opaque.