servo: Merge #12493 - Implement referrer policy delivery via noreferrer link relation (from TheKK:referrer_policy_dliver_via_rel); r=jdm

According to https://w3c.github.io/webappsec-referrer-policy/#referrer-policy-delivery, there's `<a>`, `<link>` and `<area>` could apply this delivery method. This PR contains changes for `<a>` and `<link>` **but** not `<area>`, since HTMLAreaElement is barely implemented.

We should file another issue for it.

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #11862
- [X] There are tests for these changes

Source-Repo: https://github.com/servo/servo
Source-Revision: 8a78e75d4314aa5ac770ec070a6ea7eed341e3ec

servo/components/script/document_loader.rs

@@ -7,7 +7,7 @@
 
 use dom::bindings::js::JS;
 use dom::document::Document;
-use msg::constellation_msg::PipelineId;
+use msg::constellation_msg::{PipelineId, ReferrerPolicy};
 use net_traits::{PendingAsyncLoad, AsyncResponseTarget, LoadContext};
 use net_traits::{ResourceThreads, IpcSend};
 use std::thread;
@@ -125,7 +125,8 @@ impl DocumentLoader {
     /// the future.
     pub fn prepare_async_load(&mut self,
                               load: LoadType,
-                              referrer: &Document) -> PendingAsyncLoad {
+                              referrer: &Document,
+                              referrer_policy: Option<ReferrerPolicy>) -> PendingAsyncLoad {
         let context = load.to_load_context();
         let url = load.url().clone();
         self.add_blocking_load(load);
@@ -133,7 +134,7 @@ impl DocumentLoader {
                               self.resource_threads.sender(),
                               url,
                               self.pipeline,
-                              referrer.get_referrer_policy(),
+                              referrer_policy.or(referrer.get_referrer_policy()),
                               Some(referrer.url().clone()))
     }
 
@@ -141,8 +142,9 @@ impl DocumentLoader {
     pub fn load_async(&mut self,
                       load: LoadType,
                       listener: AsyncResponseTarget,
-                      referrer: &Document) {
-        let pending = self.prepare_async_load(load, referrer);
+                      referrer: &Document,
+                      referrer_policy: Option<ReferrerPolicy>) {
+        let pending = self.prepare_async_load(load, referrer, referrer_policy);
         pending.load_async(listener)
     }