Auto merge of #26450 - PeterZhizhin:add_referrerpolicy_to_list_of_relevant_mutations, r=jdm

Add referrerpolicy to list of relevant mutations

<!-- Please describe your changes on the following line: -->

`img.referrerPolicy` change now mutates the image.
Not all `referrerPolicy` attribute changes result in an image update event.
Only valid changes are reflected.

All referrerpolicy tests inside `relevant-mutations.html` WPT test now pass.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #26388

<!-- Either: -->
- [x] These have tests:
```
./mach test-wpt tests/wpt/web-platform-tests/html/semantics/embedded-content/the-img-element/relevant-mutations.html
```

components/net_traits/lib.rs

@@ -126,6 +126,22 @@ pub enum ReferrerPolicy {
     StrictOriginWhenCrossOrigin,
 }
 
+impl ToString for ReferrerPolicy {
+    fn to_string(&self) -> String {
+        match self {
+            ReferrerPolicy::NoReferrer => "no-referrer",
+            ReferrerPolicy::NoReferrerWhenDowngrade => "no-referrer-when-downgrade",
+            ReferrerPolicy::Origin => "origin",
+            ReferrerPolicy::SameOrigin => "same-origin",
+            ReferrerPolicy::OriginWhenCrossOrigin => "origin-when-cross-origin",
+            ReferrerPolicy::UnsafeUrl => "unsafe-url",
+            ReferrerPolicy::StrictOrigin => "strict-origin",
+            ReferrerPolicy::StrictOriginWhenCrossOrigin => "strict-origin-when-cross-origin",
+        }
+        .to_string()
+    }
+}
+
 impl From<ReferrerPolicyHeader> for ReferrerPolicy {
     fn from(policy: ReferrerPolicyHeader) -> Self {
         match policy {