style: Split stylesheets.rs

This file has become quite bloated lately. This commit deletes that file in
favor of a set of submodules.

The only noticeable change apart from code move, is converting deep_clone_foo
methods into a trait.

It also unifies logic related to different style rules in the same place.

There's some missing work, specially related to font-face and counter-style, but
I think this is worth landing in the meantime.

components/style/stylesheets/media_rule.rs

@@ -0,0 +1,60 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+//! An [`@media`][media] urle.
+//!
+//! [media]: https://drafts.csswg.org/css-conditional/#at-ruledef-media
+
+use cssparser::SourceLocation;
+use media_queries::MediaList;
+use shared_lock::{DeepCloneWithLock, Locked, SharedRwLock, SharedRwLockReadGuard, ToCssWithGuard};
+use std::fmt;
+use style_traits::ToCss;
+use stylearc::Arc;
+use stylesheets::CssRules;
+
+/// An [`@media`][media] urle.
+///
+/// [media]: https://drafts.csswg.org/css-conditional/#at-ruledef-media
+#[derive(Debug)]
+pub struct MediaRule {
+    /// The list of media queries used by this media rule.
+    pub media_queries: Arc<Locked<MediaList>>,
+    /// The nested rules to this media rule.
+    pub rules: Arc<Locked<CssRules>>,
+    /// The source position where this media rule was found.
+    pub source_location: SourceLocation,
+}
+
+impl ToCssWithGuard for MediaRule {
+    // Serialization of MediaRule is not specced.
+    // https://drafts.csswg.org/cssom/#serialize-a-css-rule CSSMediaRule
+    fn to_css<W>(&self, guard: &SharedRwLockReadGuard, dest: &mut W) -> fmt::Result
+    where W: fmt::Write {
+        dest.write_str("@media ")?;
+        self.media_queries.read_with(guard).to_css(dest)?;
+        dest.write_str(" {")?;
+        for rule in self.rules.read_with(guard).0.iter() {
+            dest.write_str(" ")?;
+            rule.to_css(guard, dest)?;
+        }
+        dest.write_str(" }")
+    }
+}
+
+impl DeepCloneWithLock for MediaRule {
+    fn deep_clone_with_lock(
+        &self,
+        lock: &SharedRwLock,
+        guard: &SharedRwLockReadGuard
+    ) -> Self {
+        let media_queries = self.media_queries.read_with(guard);
+        let rules = self.rules.read_with(guard);
+        MediaRule {
+            media_queries: Arc::new(lock.wrap(media_queries.clone())),
+            rules: Arc::new(lock.wrap(rules.deep_clone_with_lock(lock, guard))),
+            source_location: self.source_location.clone(),
+        }
+    }
+}