iOS has some old #ifdefs, like the one in RenderLayerScrollableArea::showsOverflowControls(), that essentially "turn off" scrollbars in WebCore, since it assumes that they are rendered in the UI process.

However, when canUseCompositedScrolling() returns false, like inside SVG <foreignObject>, then the code still tries to paint scrollbars, which does nothing.

Instead, we should fix ScrollbarThemeIOS to return true from `usesOverlayScrollbars()` and make the iOS code closer to the macOS UI-side compositing code.

Also, at some point, we should support resizes on iOS, which requires that we run code in RenderLayerScrollableArea::paintOverflowControls.