Bug 1866277 - Fix <link rel=preload as=fetch> priority when fetchpriority disabled. r=emilio

This commit aligns the internal priority of <link rel=preload as=fetch>
when fetchpriority support is disabled with the fetchpriority=auto case
when the support is enabled. Also aligns unreachable code in
LinkRelPreloadFetch with that value.

Differential Revision: https://phabricator.services.mozilla.com/D199877

testing/web-platform/mozilla/meta/fetch/fetchpriority/fetchpriority-disabled.h2.html.ini

@@ -1,11 +1,3 @@
 [fetchpriority-disabled.h2.html]
   lsan-allowed: [mozilla::net::AddStaticElement, InitializeStaticHeaders, mozilla::net::nvFIFO::nvFIFO, mozilla::net::Http2BaseCompressor::Http2BaseCompressor]  # https://bugzilla.mozilla.org/show_bug.cgi?id=1759310
   prefs: [network.fetchpriority.enabled:false]
-  [link-initial-preload-fetch.h2.html: test different 'fetchpriority' values]
-    expected: FAIL
-
-  [link-dynamic-preload-fetch.h2.html: test different 'fetchpriority' values]
-    expected: FAIL
-
-  [link-header.h2.html?pipe=header(Link,<dummy.txt?1>; rel=preload; as=fetch; fetchpriority=low,True)|header(Link,<dummy.txt?2>; rel=preload; as=fetch; fetchpriority=high,True)|header(Link,<dummy.txt?3>; rel=preload; as=fetch; fetchpriority=auto,True)|header(Link,<dummy.txt?4>; rel=preload; as=fetch,True): test different 'fetchpriority' values]
-    expected: FAIL

uriloader/preload/PreloadService.cpp

@@ -163,7 +163,7 @@ class SupportsPriorityValueFor {
 
   static int32_t LinkRelPreloadFetch(const FetchPriority aFetchPriority) {
     if (!StaticPrefs::network_fetchpriority_enabled()) {
-      return nsISupportsPriority::PRIORITY_NORMAL;
+      return nsISupportsPriority::PRIORITY_HIGH;
     }
 
     switch (aFetchPriority) {
@@ -176,7 +176,7 @@ class SupportsPriorityValueFor {
     }
 
     MOZ_ASSERT_UNREACHABLE();
-    return nsISupportsPriority::PRIORITY_NORMAL;
+    return nsISupportsPriority::PRIORITY_HIGH;
   }
 };