Web Animations CSS: remove unnecessary class CSSValue declaration
Web Animations CSS: remove unnecessary class CSSValue declaration

The code already #includes "core/css/CSSValue.h" so no need to declare
the class a second time.

Review URL: https://chromiumcodereview.appspot.com/24072007

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@157995 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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