Bug 1331213: Bootstrap a Gecko-side Device, and track it's dirtiness manually in the per-doc data. r=heycam

The setup is quite different to Servo-land, so add a comment about the different
setup.

Also, check viewport rules when flushing stylesheets. I believe that the
previous behavior is plain wrong, though I haven't taken the time to come up
with a test case.

In any case, it doesn't hurt any of both back-ends.

MozReview-Commit-ID: 46gtTkesOsr
Signed-off-by: Emilio Cobos Álvarez <emilio@crisal.io>

components/style/media_queries.rs

@@ -19,7 +19,7 @@ pub use servo::media_queries::{Device, Expression};
 pub use gecko::media_queries::{Device, Expression};
 
 /// A type that encapsulates a media query list.
-#[derive(Debug, PartialEq)]
+#[derive(Debug)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 pub struct MediaList {
     /// The list of media queries.
@@ -66,7 +66,7 @@ impl ToCss for Qualifier {
 /// A [media query][mq].
 ///
 /// [mq]: https://drafts.csswg.org/mediaqueries/
-#[derive(Clone, Debug, PartialEq)]
+#[derive(Clone, Debug)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 pub struct MediaQuery {
     /// The qualifier for this query.