Auto merge of #13839 - servo:locked-style, r=mbrubeck

Add RwLock in more Arc’d things in stylesheets.

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CSSOM needs hold potentially multiple references (through `Arc`) to these objects, and then mutate them.

CC @upsuper
r? @mbrubeck

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/layout_thread/lib.rs

@@ -113,7 +113,7 @@ use style::media_queries::{Device, MediaType};
 use style::parallel::WorkQueueData;
 use style::parser::ParserContextExtraData;
 use style::selector_matching::Stylist;
-use style::stylesheets::{CSSRuleIteratorExt, Origin, Stylesheet, UserAgentStylesheets};
+use style::stylesheets::{Origin, Stylesheet, UserAgentStylesheets};
 use style::thread_state;
 use style::timer::Timer;
 use style::workqueue::WorkQueue;
@@ -354,21 +354,21 @@ fn add_font_face_rules(stylesheet: &Stylesheet,
                        outstanding_web_fonts_counter: &Arc<AtomicUsize>) {
     if opts::get().load_webfonts_synchronously {
         let (sender, receiver) = ipc::channel().unwrap();
-        for font_face in stylesheet.effective_rules(&device).font_face() {
+        stylesheet.effective_font_face_rules(&device, |font_face| {
             let effective_sources = font_face.effective_sources();
             font_cache_thread.add_web_font(font_face.family.clone(),
                                            effective_sources,
                                            sender.clone());
             receiver.recv().unwrap();
-        }
+        })
     } else {
-        for font_face in stylesheet.effective_rules(&device).font_face() {
+        stylesheet.effective_font_face_rules(&device, |font_face| {
             let effective_sources = font_face.effective_sources();
             outstanding_web_fonts_counter.fetch_add(1, Ordering::SeqCst);
             font_cache_thread.add_web_font(font_face.family.clone(),
                                           effective_sources,
                                           (*font_cache_sender).clone());
-        }
+        })
     }
 }