Made CSSPrimitiveValue::getString() only work for String types

Source/core/css/resolver/StyleBuilderConverter.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
  * copyright notice, this list of conditions and the following disclaimer
  * in the documentation and/or other materials provided with the
  * distribution.
  *     * Neither the name of Google Inc. nor the names of its
@@ skipping 139 matching lines @@
     ASSERT_NOT_REACHED();
     return 0;
 }
 
 template <CSSValueID IdForNone>
 AtomicString StyleBuilderConverter::convertString(StyleResolverState&, CSSValue* value)
 {
     CSSPrimitiveValue* primitiveValue = toCSSPrimitiveValue(value);
     if (primitiveValue->getValueID() == IdForNone)
         return nullAtom;
+    ASSERT(primitiveValue->isString());
     return AtomicString(primitiveValue->getStringValue());
 }
 
 } // namespace blink
 
 #endif