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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ports/cef/window.rs

@@ -484,6 +484,10 @@ impl WindowMethods for Window {
         }
     }
 
+    fn allow_navigation(&self, _: ServoUrl) -> bool {
+        true
+    }
+
     fn supports_clipboard(&self) -> bool {
         false
     }

ports/glutin/window.rs

@@ -1107,6 +1107,10 @@ impl WindowMethods for Window {
         }
     }
 
+    fn allow_navigation(&self, _: ServoUrl) -> bool {
+        true
+    }
+
     fn supports_clipboard(&self) -> bool {
         false
     }