Auto merge of #17083 - danielj41:javascript-url-global-3, r=jdm

"javascript:" urls: execute in correct global scope

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#### Summary

This pull request makes `javascript:` urls execute in the correct global scope.

#### Example

```html
<script> var x = 4; </script>

<!-- this branch: logs "4" -->
<!-- master: undefined variable error -->
<a href="javascript:console.log(x)">link</a>
```

#### Questions

I'm new to servo and rust, so I'm unsure about these changes. In particular:
  * What's the appropriate place to evaluate the js?
    * I moved it to `handle_navigate`, but I'm not sure if this will catch all occurrences of `javascript:` urls. I also don't know if this will execute in the correct thread and the correct window.
  * What should I do with the result of the js evaluation?
    * I just ignored it. The previous behavior displayed it as the content of a new page load.

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #15147, #16718

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- [x] There are tests for these changes

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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,
         }

components/script_traits/script_msg.rs

@@ -99,6 +99,8 @@ pub enum ScriptMsg {
     /// A new load has been requested, with an option to replace the current entry once loaded
     /// instead of adding a new entry.
     LoadUrl(LoadData, bool),
+    /// Abort loading after sending a LoadUrl message.
+    AbortLoadUrl,
     /// Post a message to the currently active window of a given browsing context.
     PostMessage(BrowsingContextId, Option<ImmutableOrigin>, Vec<u8>),
     /// Dispatch a mozbrowser event to the parent of a mozbrowser iframe.