Preserve sourceURL comment on style sheets

In addition to the sourceMappingURL comment, there is a second special
comment, "sourceURL", that can be used to set the "display name" of a
style sheet for developer tools.  This name is also used as the base
URL for the source-map URL resolution algorithm.  sourceURL is
described here:
https://blog.getfirebug.com/2009/08/11/give-your-eval-a-name-with-sourceurl/
The devtools feature bug is here:
https://bugzilla.mozilla.org/show_bug.cgi?id=880831

This patch changes servo to preserve and expose this value for use in M-C.

tests/unit/gfx/Cargo.toml

@@ -10,7 +10,7 @@ path = "lib.rs"
 doctest = false
 
 [dependencies]
-cssparser = "0.21.0"
+cssparser = "0.21.1"
 gfx = {path = "../../../components/gfx"}
 ipc-channel = "0.8"
 style = {path = "../../../components/style"}

tests/unit/style/Cargo.toml

@@ -12,7 +12,7 @@ doctest = false
 [dependencies]
 byteorder = "1.0"
 app_units = "0.5"
-cssparser = "0.21.0"
+cssparser = "0.21.1"
 euclid = "0.15"
 html5ever = "0.19"
 parking_lot = "0.4"

tests/unit/style/stylesheets.rs

@@ -249,6 +249,7 @@ fn test_parse_stylesheet() {
                 })))
             ], &stylesheet.shared_lock),
             source_map_url: RwLock::new(None),
+            source_url: RwLock::new(None),
         },
         media: Arc::new(stylesheet.shared_lock.wrap(MediaList::empty())),
         shared_lock: stylesheet.shared_lock.clone(),
@@ -377,3 +378,22 @@ fn test_source_map_url() {
         assert_eq!(*url_opt, test.1);
     }
 }
+
+#[test]
+fn test_source_url() {
+    let tests = vec![
+        ("", None),
+        ("/*# sourceURL=something */", Some("something".to_string())),
+    ];
+
+    for test in tests {
+        let url = ServoUrl::parse("about::test").unwrap();
+        let lock = SharedRwLock::new();
+        let media = Arc::new(lock.wrap(MediaList::empty()));
+        let stylesheet = Stylesheet::from_str(test.0, url.clone(), Origin::UserAgent, media, lock,
+                                              None, &CSSErrorReporterTest, QuirksMode::NoQuirks,
+                                              0);
+        let url_opt = stylesheet.contents.source_url.read();
+        assert_eq!(*url_opt, test.1);
+    }
+}

tests/unit/stylo/Cargo.toml

@@ -13,7 +13,7 @@ doctest = false
 
 [dependencies]
 atomic_refcell = "0.1"
-cssparser = "0.21.0"
+cssparser = "0.21.1"
 env_logger = "0.4"
 euclid = "0.15"
 geckoservo = {path = "../../../ports/geckolib"}