Adjust Screen's derived Oilpan type after r172152
Adjust Screen's derived Oilpan type after r172152

Following r172152, Screen is no longer an EventTarget. Hence we can
simplify it to derive from GarbageCollectedFinalized only.

R=
BUG=162827

Review URL: https://codereview.chromium.org/246253008

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@172289 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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