Bug 1303229: Get the proper viewport size for stylo. r=heycam

At least until we support scrollbars properly, this size is going to be the
correct one. I've left a TODO to grab the proper one once we support it.

This allows to trivially test viewport units for now.

MozReview-Commit-ID: JdaZ6WlZ2C6
Signed-off-by: Emilio Cobos Álvarez <emilio@crisal.io>

components/style/context.rs

@@ -62,9 +62,6 @@ 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>,
 
@@ -92,6 +89,13 @@ 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.