Auto merge of #16968 - HeyZoos:stylist-accessors, r=emilio

Stylist accessors

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Add accessor methods for the `device` and `ruleset` fields in the `Stylist` struct.

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- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #16857 (github issue number if applicable).
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- [X] There are tests for these changes

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components/style/context.rs

@@ -150,7 +150,7 @@ pub struct SharedStyleContext<'a> {
 impl<'a> SharedStyleContext<'a> {
     /// Return a suitable viewport size in order to be used for viewport units.
     pub fn viewport_size(&self) -> Size2D<Au> {
-        self.stylist.device.au_viewport_size()
+        self.stylist.device().au_viewport_size()
     }
 }