Add UseCounter for rel=stylesheet link headers

PreloadHelper::LoadLinksFromHeader is currently the only time we
consider loading link headers, so in order to count how many of these
link headers are not rel=preload, but rel=stylesheet, this seems to be
the best place to perform a count.

Bug: 19237
Change-Id: I1c5cb5eb2200875a56714484f40e955b27d13746
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1710505Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Dominic Farolino <dom@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679551}

third_party/blink/public/mojom/web_feature/web_feature.mojom

@@ -2353,6 +2353,7 @@ enum WebFeature {
   kCSSValueAppearanceMeterForOthersRendered = 2967,
   kSVGSMILDiscardElementParsed = 2968,
   kSVGSMILDiscardElementTriggered = 2969,
+  kLinkHeaderStylesheet = 2970,
 
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.

third_party/blink/renderer/core/loader/preload_helper.cc

@@ -528,6 +528,13 @@ void PreloadHelper::LoadLinksFromHeader(
     if (media_policy == kOnlyLoadNonMedia && header.IsViewportDependent())
       continue;
 
+    // TODO(domfarolino): Remove the following UseCounter-related lines when
+    // data on link stylesheet headers has been collected. See
+    // https://crbug.com/19237.
+    DCHECK_EQ(header.Rel(), header.Rel().DeprecatedLower());
+    if (header.Rel() == "stylesheet")
+      UseCounter::Count(document, WebFeature::kLinkHeaderStylesheet);
+
     LinkLoadParameters params(header, base_url);
     if (alternate_resource_info && params.rel.IsLinkPreload()) {
       DCHECK(document);

tools/metrics/histograms/enums.xml

@@ -23955,6 +23955,7 @@ Called by update_net_error_codes.py.-->
   <int value="2967" label="CSSValueAppearanceMeterForOthersRendered"/>
   <int value="2968" label="SVGSMILDiscardElementParsed"/>
   <int value="2969" label="SVGSMILDiscardElementTriggered"/>
+  <int value="2970" label="LinkHeaderStylesheet"/>
 </enum>
 
 <enum name="FeaturePolicyAllowlistType">