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.

components/style/Cargo.toml

@@ -37,7 +37,7 @@ atomic_refcell = "0.1"
 bitflags = "0.7"
 byteorder = "1.0"
 cfg-if = "0.1.0"
-cssparser = "0.21.0"
+cssparser = "0.21.1"
 encoding = {version = "0.2", optional = true}
 euclid = "0.15"
 fallible = { path = "../fallible" }

components/style/gecko/generated/bindings.rs

@@ -1970,6 +1970,10 @@ extern "C" {
                                                 RawServoStyleSheetContentsBorrowed,
                                             result: *mut nsAString);
 }
+extern "C" {
+    pub fn Servo_StyleSheet_GetSourceURL(sheet: RawServoStyleSheetContentsBorrowed,
+                                         result: *mut nsAString);
+}
 extern "C" {
     pub fn Servo_StyleSheet_GetOrigin(sheet:
                                           RawServoStyleSheetContentsBorrowed)

components/style/stylesheets/stylesheet.rs

@@ -62,6 +62,8 @@ pub struct StylesheetContents {
     pub quirks_mode: QuirksMode,
     /// This stylesheet's source map URL.
     pub source_map_url: RwLock<Option<String>>,
+    /// This stylesheet's source URL.
+    pub source_url: RwLock<Option<String>>,
 }
 
 impl StylesheetContents {
@@ -78,7 +80,7 @@ impl StylesheetContents {
         line_number_offset: u32
     ) -> Self {
         let namespaces = RwLock::new(Namespaces::default());
-        let (rules, source_map_url) = Stylesheet::parse_rules(
+        let (rules, source_map_url, source_url) = Stylesheet::parse_rules(
             css,
             &url_data,
             origin,
@@ -97,6 +99,7 @@ impl StylesheetContents {
             namespaces: namespaces,
             quirks_mode: quirks_mode,
             source_map_url: RwLock::new(source_map_url),
+            source_url: RwLock::new(source_url),
         }
     }
 
@@ -146,6 +149,7 @@ impl DeepCloneWithLock for StylesheetContents {
             url_data: RwLock::new((*self.url_data.read()).clone()),
             namespaces: RwLock::new((*self.namespaces.read()).clone()),
             source_map_url: RwLock::new((*self.source_map_url.read()).clone()),
+            source_url: RwLock::new((*self.source_map_url.read()).clone()),
         }
     }
 }
@@ -314,7 +318,7 @@ impl Stylesheet {
         where R: ParseErrorReporter
     {
         let namespaces = RwLock::new(Namespaces::default());
-        let (rules, source_map_url) =
+        let (rules, source_map_url, source_url) =
             Stylesheet::parse_rules(
                 css,
                 &url_data,
@@ -337,6 +341,7 @@ impl Stylesheet {
         let mut guard = existing.shared_lock.write();
         *existing.contents.rules.write_with(&mut guard) = CssRules(rules);
         *existing.contents.source_map_url.write() = source_map_url;
+        *existing.contents.source_url.write() = source_url;
     }
 
     fn parse_rules<R: ParseErrorReporter>(
@@ -349,7 +354,7 @@ impl Stylesheet {
         error_reporter: &R,
         quirks_mode: QuirksMode,
         line_number_offset: u32
-    ) -> (Vec<CssRule>, Option<String>) {
+    ) -> (Vec<CssRule>, Option<String>, Option<String>) {
         let mut rules = Vec::new();
         let mut input = ParserInput::new_with_line_number_offset(css, line_number_offset);
         let mut input = Parser::new(&mut input);
@@ -399,7 +404,8 @@ impl Stylesheet {
         }
 
         let source_map_url = input.current_source_map_url().map(String::from);
-        (rules, source_map_url)
+        let source_url = input.current_source_url().map(String::from);
+        (rules, source_map_url, source_url)
     }
 
     /// Creates an empty stylesheet and parses it with a given base url, origin