HTML Imports: Add an UseCounter for <link rel=import async>

Source/core/html/imports/HTMLImportsController.cpp

Index: Source/core/html/imports/HTMLImportsController.cpp
diff --git a/Source/core/html/imports/HTMLImportsController.cpp b/Source/core/html/imports/HTMLImportsController.cpp
index faf0df9ca7462ee70d92d7dcb8cde8af9056527e..8afd0dd74491946549d3ace2e2a616aca2e32ecb 100644
--- a/Source/core/html/imports/HTMLImportsController.cpp
+++ b/Source/core/html/imports/HTMLImportsController.cpp
@@ -91,6 +91,9 @@ static bool makesCycle(HTMLImport* parent, const KURL& url)
 HTMLImportChild* HTMLImportsController::createChild(const KURL& url, HTMLImportLoader* loader, HTMLImport* parent, HTMLImportChildClient* client)
 {
     HTMLImport::SyncMode mode = client->isSync() && !makesCycle(parent, url) ? HTMLImport::Sync : HTMLImport::Async;
+    if (mode == HTMLImport::Async)
+        UseCounter::count(root()->document(), UseCounter::HTMLImportsAsyncAttribute);
+
     OwnPtrWillBeRawPtr<HTMLImportChild> child = adoptPtrWillBeNoop(new HTMLImportChild(url, loader, mode));
     child->setClient(client);
     parent->appendImport(child.get());