Cache effective media query results.

This patch also makes RulesIterator not iterate over rules for which we don't
process nested rules.

There's nothing depending on this behavior right now afaik, and this will make
us duplicate less code in following patches.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1357461
MozReview-Commit-ID: CaMFQtAVnJF

components/style/stylesheet_set.rs

@@ -175,6 +175,11 @@ impl StylesheetSet {
         self.dirty = false;
         self.invalidations.flush(document_element);
 
+        self.iter()
+    }
+
+    /// Returns an iterator over the current list of stylesheets.
+    pub fn iter(&self) -> StylesheetIterator {
         StylesheetIterator(self.entries.iter())
     }