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: svn://svn.chromium.org/blink/trunk@172289 bbb929c8-8fbe-4397-9dbb-9b2b20218538
1 file changed