Auto merge of #15795 - paulrouget:follow-link, r=asajeffrey

Let the embedder decide if servo should follow a link or not

We want to give a chance to the embedder to handle a link itself.
Is it a problem that this will add a round trip to the main thread every time `load_url` is called?

---
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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 #15655

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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because I'm not sure how to test that

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components/compositing/compositor.rs

@@ -544,6 +544,13 @@ impl<Window: WindowMethods> IOCompositor<Window> {
                 self.window.load_end(back, forward, root);
             }
 
+            (Msg::AllowNavigation(url, response_chan), ShutdownState::NotShuttingDown) => {
+                let allow = self.window.allow_navigation(url);
+                if let Err(e) = response_chan.send(allow) {
+                    warn!("Failed to send allow_navigation result ({}).", e);
+                }
+            }
+
             (Msg::DelayedCompositionTimeout(timestamp), ShutdownState::NotShuttingDown) => {
                 if let CompositionRequest::DelayedComposite(this_timestamp) =
                     self.composition_request {