Prefetch @import files from CSS files.

Source/core/fetch/StyleSheetResourceClient.h

Index: Source/core/fetch/StyleSheetResourceClient.h
diff --git a/Source/core/fetch/StyleSheetResourceClient.h b/Source/core/fetch/StyleSheetResourceClient.h
index bde2093d3f18baa558f600352c837cad70c69471..eae9021dc19483462ab0f0d62299d8ab098296af 100644
--- a/Source/core/fetch/StyleSheetResourceClient.h
+++ b/Source/core/fetch/StyleSheetResourceClient.h
@@ -40,6 +40,7 @@ public:
     virtual ResourceClientType resourceClientType() const override final { return expectedType(); }
     virtual void setCSSStyleSheet(const String& /* href */, const KURL& /* baseURL */, const String& /* charset */, const CSSStyleSheetResource*) { }
     virtual void setXSLStyleSheet(const String& /* href */, const KURL& /* baseURL */, const String& /* sheet */) { }
+    virtual void dataReceived(const CSSStyleSheetResource* /* cachedStyleSheet */) { }
 };
 
 }