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

TEST=none
BUG=383749
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/334263005

git-svn-id: svn://svn.chromium.org/blink/trunk@176240 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/core/frame/UseCounter.h

@@ -460,6 +460,7 @@ public:
         EditingApplePasteAsQuotation = 460,
         EditingAppleStyleSpanClass = 461,
         EditingAppleTabSpanClass = 462,
+        HTMLImportsAsyncAttribute = 463,
         // Add new features immediately above this line. Don't change assigned
         // numbers of any item, and don't reuse removed slots.
         // Also, run update_use_counter_feature_enum.py in chromium/src/tools/metrics/histograms/

third_party/WebKit/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());