Auto merge of #15913 - emilio:viewport-size, r=heycam,hiro

Use the proper viewport size for stylo

Reviewed upstream at [bug 1303229](https://bugzil.la/1303229).

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components/style/context.rs

@@ -63,9 +63,6 @@ pub enum QuirksMode {
 /// There's exactly one of these during a given restyle traversal, and it's
 /// shared among the worker threads.
 pub struct SharedStyleContext {
-    /// The current viewport size.
-    pub viewport_size: Size2D<Au>,
-
     /// The CSS selector stylist.
     pub stylist: Arc<Stylist>,
 
@@ -93,6 +90,13 @@ pub struct SharedStyleContext {
     pub default_computed_values: Arc<ComputedValues>,
 }
 
+impl SharedStyleContext {
+    /// 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()
+    }
+}
+
 /// Information about the current element being processed. We group this together
 /// into a single struct within ThreadLocalStyleContext so that we can instantiate
 /// and destroy it easily at the beginning and end of element processing.