[Code health] Use defined type in MediaValueCached::CrossThreadCopier

In this template, we defined MediaValuesCached::MediaValuesCachedData to
be Type, so after that definition, we should just use Type instead of
MediaValuesCached::MediaValuesCachedData.

This CL does not introduce behavior change.

Bug: None
Change-Id: I7337cf2a1d87aeecc8ff1048e1a666753b0166c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1643887Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666073}

third_party/blink/renderer/core/css/media_values_cached.h

@@ -113,9 +113,7 @@ class CORE_EXPORT MediaValuesCached final : public MediaValues {
 template <>
 struct CrossThreadCopier<MediaValuesCached::MediaValuesCachedData> {
   typedef MediaValuesCached::MediaValuesCachedData Type;
-  static Type Copy(const MediaValuesCached::MediaValuesCachedData& data) {
-    return data.DeepCopy();
-  }
+  static Type Copy(const Type& data) { return data.DeepCopy(); }
 };
 
 }  // namespace blink