Assert expected values in StyleBuilderConverter::ConvertElementReference

This converter converts "<url> | none", so add DCHECKs to that effect.

Bug: 109212
Change-Id: I35f7629a1d233ed5a6e884af6538441e5a4a8d26
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2587170Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#836635}

third_party/blink/renderer/core/css/resolver/style_builder_converter.cc

@@ -117,12 +117,15 @@ scoped_refptr<StyleReflection> StyleBuilderConverter::ConvertBoxReflect(
 scoped_refptr<StyleSVGResource> StyleBuilderConverter::ConvertElementReference(
     StyleResolverState& state,
     const CSSValue& value) {
-  const auto* url_value = DynamicTo<cssvalue::CSSURIValue>(value);
-  if (!url_value)
+  if (auto* identifier_value = DynamicTo<CSSIdentifierValue>(value)) {
+    DCHECK_EQ(identifier_value->GetValueID(), CSSValueID::kNone);
     return nullptr;
+  }
+
+  const auto& url_value = To<cssvalue::CSSURIValue>(value);
   SVGResource* resource =
-      state.GetElementStyleResources().GetSVGResourceFromValue(*url_value);
-  return StyleSVGResource::Create(resource, url_value->ValueForSerialization());
+      state.GetElementStyleResources().GetSVGResourceFromValue(url_value);
+  return StyleSVGResource::Create(resource, url_value.ValueForSerialization());
 }
 
 LengthBox StyleBuilderConverter::ConvertClip(StyleResolverState& state,