Expose user agent option to DOM navigator interface.

components/script/dom/navigator.rs

@@ -51,6 +51,10 @@ impl<'a> NavigatorMethods for JSRef<'a, Navigator> {
     fn Platform(self) -> DOMString {
         NavigatorInfo::Platform()
     }
+
+    fn UserAgent(self) -> DOMString {
+        NavigatorInfo::UserAgent()
+    }
 }
 
 impl Reflectable for Navigator {

components/script/dom/navigatorinfo.rs

@@ -3,6 +3,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 use servo_util::str::DOMString;
+use servo_util::opts;
 
 pub struct NavigatorInfo;
 
@@ -26,4 +27,11 @@ impl NavigatorInfo {
     pub fn Platform() -> DOMString {
         "".to_string()
     }
+
+    pub fn UserAgent() -> DOMString {
+        match opts::get().user_agent {
+            Some(ref user_agent) => user_agent.clone(),
+            None => "".to_string(),
+        }
+    }
 }

components/script/dom/webidls/Navigator.webidl

@@ -23,5 +23,5 @@ interface NavigatorID {
   readonly attribute DOMString platform;
   readonly attribute DOMString product; // constant "Gecko"
   boolean taintEnabled(); // constant false
-  //readonly attribute DOMString userAgent;
+  readonly attribute DOMString userAgent;
 };

components/script/dom/workernavigator.rs

@@ -51,6 +51,10 @@ impl<'a> WorkerNavigatorMethods for JSRef<'a, WorkerNavigator> {
     fn Platform(self) -> DOMString {
         NavigatorInfo::Platform()
     }
+
+    fn UserAgent(self) -> DOMString {
+        NavigatorInfo::UserAgent()
+    }
 }
 
 impl Reflectable for WorkerNavigator {