"javascript:" urls: run in correct global

Make some changes to javascript scheme url evaluation to bring it
closer to
https://html.spec.whatwg.org/multipage/browsing-the-web.html#navigating-across-documents

- Evaluate the js before the page load so that it happens in the
  correct `window` global.
- If the result is not a string, the response should be a 204.

This required saving some data in load_data, since it's not
possible to modify the response at the point where we're evaluating
the js.

components/script_traits/lib.rs

@@ -146,12 +146,24 @@ pub struct LoadData {
     pub headers: Headers,
     /// The data.
     pub data: Option<Vec<u8>>,
+    /// The result of evaluating a javascript scheme url.
+    pub js_eval_result: Option<JsEvalResult>,
     /// The referrer policy.
     pub referrer_policy: Option<ReferrerPolicy>,
     /// The referrer URL.
     pub referrer_url: Option<ServoUrl>,
 }
 
+/// The result of evaluating a javascript scheme url.
+#[derive(Clone, Debug, Deserialize, Serialize)]
+pub enum JsEvalResult {
+    /// The js evaluation had a non-string result, 204 status code.
+    /// https://html.spec.whatwg.org/multipage/#navigate 12.11
+    NoContent,
+    /// The js evaluation had a string result.
+    Ok(Vec<u8>)
+}
+
 impl LoadData {
     /// Create a new `LoadData` object.
     pub fn new(url: ServoUrl,
@@ -165,6 +177,7 @@ impl LoadData {
             method: Method::Get,
             headers: Headers::new(),
             data: None,
+            js_eval_result: None,
             referrer_policy: referrer_policy,
             referrer_url: referrer_url,
         }