Added <link rel=preconnect> crossorigin attribute

Source/core/html/CrossOriginAttribute.h

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CrossOriginAttribute_h
+#define CrossOriginAttribute_h
+
+#include "public/platform/WebCrossOriginAttribute.h"
+#include "wtf/text/WTFString.h"
+
+namespace blink {
+
+CrossOriginAttributeValue crossOriginAttributeValue(const String& value)
+{
+    if (value.isNull())
+        return CrossOriginAttributeNotSet;
+    if (equalIgnoringCase(value, "use-credentials"))
+        return CrossOriginAttributeUseCredentials;
+    return CrossOriginAttributeAnonymous;

[Mike West, 2015/05/11 12:53:35]

Is this the behavior we current exhibit? That is, if I have
`crossorigin="ljkahsfgkjbsdnfk;j"`, what happens?

[sof, 2015/05/11 12:57:30]

The invalid value default for the crossorigin attribute is "anonymous", 

 https://html.spec.whatwg.org/#cors-settings-attribute

so this looks consistent with that.

+}
+
+} // namespace blink
+
+#endif
+
+