Bug 1844185 - Add a pref to optionally disable preconnect r=necko-reviewers,kershaw

We want to test with and without this feature, so expose it via pref.

Differential Revision: https://phabricator.services.mozilla.com/D184150

dom/base/Document.cpp

@@ -12469,6 +12469,10 @@ void Document::MaybePreLoadImage(nsIURI* aUri,
 }
 
 void Document::MaybePreconnect(nsIURI* aOrigURI, mozilla::CORSMode aCORSMode) {
+  if (!StaticPrefs::network_preconnect()) {
+    return;
+  }
+
   NS_MutateURI mutator(aOrigURI);
   if (NS_FAILED(mutator.GetStatus())) {
     return;

modules/libpref/init/StaticPrefList.yaml

@@ -11704,6 +11704,13 @@
   value: true
   mirror: always
 
+# Enables `<link rel="preconnect">` tag and `Link: rel=preconnect` response header
+# handling.
+- name: network.preconnect
+  type: RelaxedAtomicBool
+  value: true
+  mirror: always
+
 # Enables `<link rel="modulepreload">` tag and `Link: rel=modulepreload`
 # response header handling.
 - name: network.modulepreload