Re-enable test: fixed-position-composited-page-scale.html

A fix for this has been landed as
https://src.chromium.org/viewvc/chrome/?revision=253909,
re-enabling the affected tests.

TBR=alph@chromium.org
NOTRY=true

BUG=347491

Review URL: https://codereview.chromium.org/183183006

git-svn-id: svn://svn.chromium.org/blink/trunk@168056 bbb929c8-8fbe-4397-9dbb-9b2b20218538