Credential manager: Introduce platform interface.

public/platform/WebCredentialManagerError.h

+// Copyright 2014 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 WebCredentialManagerError_h
+#define WebCredentialManagerError_h
+
+#include "public/platform/WebString.h"
+
+namespace blink {
+
+struct WebCredentialManagerError {
+    // FIXME: This is a placeholder list of error conditions. We'll likely expand the
+    // list as the API evolves.
+    enum ErrorType {
+        ErrorTypeDisabled = 0,
+        ErrorTypeUnknown,
+        ErrorTypeLast = ErrorTypeUnknown
+    };
+
+    WebCredentialManagerError(ErrorType type, WebString message)
+        : errorType(type)
+        , errorMessage(message)
+    {
+    }
+
+    ErrorType errorType;
+    WebString errorMessage;
+};
+
+} // namespace blink
+
+#endif