Bug 1865610: part 1) Add partial support for the "fetchpriority" attribute for loading style sheets from <link> elements or Link header fields. r=smaug,emilio,valentin

Includes only support for dynamically loading style sheets from `<link>`
elements and `Link` header fields.
The remaining support, including preloading, is completed in part 3.

Differential Revision: https://phabricator.services.mozilla.com/D191745

dom/html/HTMLLinkElement.cpp

@@ -389,6 +389,7 @@ Maybe<LinkStyle::SheetInfo> HTMLLinkElement::GetStyleSheetInfo() {
       alternate ? HasAlternateRel::Yes : HasAlternateRel::No,
       IsInline::No,
       mExplicitlyEnabled ? IsExplicitlyEnabled::Yes : IsExplicitlyEnabled::No,
+      GetFetchPriority(),
   });
 }