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

Source/core/css/resolver/StyleBuilderConverter.h

Index: Source/core/css/resolver/StyleBuilderConverter.h
diff --git a/Source/core/css/resolver/StyleBuilderConverter.h b/Source/core/css/resolver/StyleBuilderConverter.h
index 6f11b7f098b9cd56f6a7d01415b9fc4bf7bd32f3..b6bfd25be0f9a706d9b24689605f0da1f35e10a0 100644
--- a/Source/core/css/resolver/StyleBuilderConverter.h
+++ b/Source/core/css/resolver/StyleBuilderConverter.h
@@ -157,6 +157,7 @@ AtomicString StyleBuilderConverter::convertString(StyleResolverState&, CSSValue*
     CSSPrimitiveValue* primitiveValue = toCSSPrimitiveValue(value);
     if (primitiveValue->getValueID() == IdForNone)
         return nullAtom;
+    ASSERT(primitiveValue->isString());
     return AtomicString(primitiveValue->getStringValue());
 }