Add a use counter for SXG prefetch

Change-Id: Ib180117dea189876c2e095a114277b6e2d1ba6eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1935972
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kunihiko Sakamoto <ksakamoto@chromium.org>
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719143}

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

@@ -2481,6 +2481,7 @@ enum WebFeature {
   kV8PortalHost_PostMessage_Method = 3100,
   kV8PortalActivateEvent_Data_AttributeGetter = 3101,
   kV8PortalActivateEvent_AdoptPredecessor_Method = 3102,
+  kLinkRelPrefetchForSignedExchanges = 3103,
 
   // 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/resource_load_observer_for_frame.cc

@@ -145,19 +145,23 @@ void ResourceLoadObserverForFrame::DidReceiveResponse(
                                                response.RemoteIPAddress());
 
   std::unique_ptr<AlternateSignedExchangeResourceInfo> alternate_resource_info;
-  if (RuntimeEnabledFeatures::SignedExchangeSubresourcePrefetchEnabled(
-          &frame_or_imported_document_->GetDocument()) &&
-      response.IsSignedExchangeInnerResponse() &&
-      resource->GetType() == ResourceType::kLinkPrefetch &&
-      resource->LastResourceResponse()) {
-    // If this is a prefetch for a SXG, see if the outer response (which must be
-    // the last response in the redirect chain) had provided alternate links for
-    // the prefetch.
-    alternate_resource_info =
-        AlternateSignedExchangeResourceInfo::CreateIfValid(
-            resource->LastResourceResponse()->HttpHeaderField(
-                http_names::kLink),
-            response.HttpHeaderField(http_names::kLink));
+
+  // See if this is a prefetch for a SXG.
+  if (response.IsSignedExchangeInnerResponse() &&
+      resource->GetType() == ResourceType::kLinkPrefetch) {
+    CountUsage(WebFeature::kLinkRelPrefetchForSignedExchanges);
+
+    if (RuntimeEnabledFeatures::SignedExchangeSubresourcePrefetchEnabled(
+            &frame_or_imported_document_->GetDocument()) &&
+        resource->LastResourceResponse()) {
+      // See if the outer response (which must be the last response in
+      // the redirect chain) had provided alternate links for the prefetch.
+      alternate_resource_info =
+          AlternateSignedExchangeResourceInfo::CreateIfValid(
+              resource->LastResourceResponse()->HttpHeaderField(
+                  http_names::kLink),
+              response.HttpHeaderField(http_names::kLink));
+    }
   }
 
   PreloadHelper::CanLoadResources resource_loading_policy =

tools/metrics/histograms/enums.xml

@@ -25590,6 +25590,7 @@ Called by update_net_error_codes.py.-->
   <int value="3100" label="V8PortalHost_PostMessage_Method"/>
   <int value="3101" label="V8PortalActivateEvent_Data_AttributeGetter"/>
   <int value="3102" label="V8PortalActivateEvent_AdoptPredecessor_Method"/>
+  <int value="3103" label="LinkRelPrefetchForSignedExchanges"/>
 </enum>
 
 <enum name="FeaturePolicyAllowlistType">