Add a use counter for Link rel=serviceworker headers.

third_party/WebKit/Source/core/loader/LinkLoader.cpp

Index: third_party/WebKit/Source/core/loader/LinkLoader.cpp
diff --git a/third_party/WebKit/Source/core/loader/LinkLoader.cpp b/third_party/WebKit/Source/core/loader/LinkLoader.cpp
index e29e40a59b3c7e14ed46d354dc0fb1da7e76fd4b..f80e0da7f0ef0f59a2ed7198bb07be442452c28f 100644
--- a/third_party/WebKit/Source/core/loader/LinkLoader.cpp
+++ b/third_party/WebKit/Source/core/loader/LinkLoader.cpp
@@ -412,6 +412,9 @@ void LinkLoader::loadLinksFromHeader(
                       LinkCalledFromHeader, errorOccurred, viewportDescription,
                       ReferrerPolicyDefault);
     }
+    if (relAttribute.isServiceWorker()) {
+      UseCounter::count(*document, UseCounter::LinkHeaderServiceWorker);
+    }
     // TODO(yoav): Add more supported headers as needed.
   }
 }