Remove left-over DEFINE_TYPE_CASTS use from css_unsupported_style_value.h

This is a follow up of https://crrev.com/c/1496217

TBR=jbroman@chromium.org

BUG=891908

Change-Id: Ib3a86b00c39ee0c0cc7081fbd87038d3a03c8cde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1509192Reviewed-by: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#638688}

third_party/blink/renderer/core/css/cssom/css_unsupported_style_value.h

@@ -67,14 +67,6 @@ class CORE_EXPORT CSSUnsupportedStyleValue final : public CSSStyleValue {
   DISALLOW_COPY_AND_ASSIGN(CSSUnsupportedStyleValue);
 };
 
-DEFINE_TYPE_CASTS(CSSUnsupportedStyleValue,
-                  CSSStyleValue,
-                  value,
-                  value->GetType() ==
-                      CSSStyleValue::StyleValueType::kUnknownType,
-                  value.GetType() ==
-                      CSSStyleValue::StyleValueType::kUnknownType);
-
 template <>
 struct DowncastTraits<CSSUnsupportedStyleValue> {
   static bool AllowFrom(const CSSStyleValue& value) {