Bug 1784496 - The necko consumers should use the priority's incremental flag r=necko-reviewers,kershaw

This patch sets the incremental flag on document and image channels

Differential Revision: https://phabricator.services.mozilla.com/D209235

devtools/client/netmonitor/test/browser_net_copy_headers.js

@@ -47,7 +47,7 @@ add_task(async function () {
     "Accept-Encoding: gzip, deflate",
     "Connection: keep-alive",
     "Upgrade-Insecure-Requests: 1",
-    "Priority: u=1",
+    "Priority: u=1, i",
     "Pragma: no-cache",
     "Cache-Control: no-cache",
   ].join("\n");

docshell/base/nsDocShell.cpp

@@ -9825,6 +9825,9 @@ nsIPrincipal* nsDocShell::GetInheritedPrincipal(
       nsCOMPtr<nsIClassOfService> cos(do_QueryInterface(channel));
       if (cos) {
         cos->AddClassFlags(nsIClassOfService::UrgentStart);
+        if (StaticPrefs::dom_document_priority_incremental()) {
+          cos->SetIncremental(true);
+        }
       }
     }
   }

image/imgLoader.cpp

@@ -2472,6 +2472,9 @@ nsresult imgLoader::LoadImage(
       if (aUseUrgentStartForChannel && !aLinkPreload) {
         cos->AddClassFlags(nsIClassOfService::UrgentStart);
       }
+      if (StaticPrefs::image_priority_incremental()) {
+        cos->SetIncremental(true);
+      }
 
       if (StaticPrefs::network_http_tailing_enabled() &&
           aContentPolicyType == nsIContentPolicy::TYPE_INTERNAL_IMAGE_FAVICON) {

modules/libpref/init/StaticPrefList.yaml

@@ -2668,6 +2668,13 @@
   value: false
   mirror: always
 
+# Whether to set the incremental flag on the top level document's
+# priority header
+- name: dom.document_priority.incremental
+  type: RelaxedAtomicBool
+  value: true
+  mirror: always
+
 # Allow the content process to create a File from a path. This is allowed just
 # on parent process, on 'file' Content process, or for testing.
 - name: dom.file.createInChild
@@ -7192,6 +7199,12 @@
   value: false
   mirror: always
 
+# Whether to set the incremental attribute in the Priority header for images
+- name: image.priority.incremental
+  type: RelaxedAtomicBool
+  value: true
+  mirror: always
+
 # Whether we attempt to decode AVIF images or not.
 - name: image.avif.enabled
   type: RelaxedAtomicBool