Remove duplicated entries from 'platform_test_files' list.

Found while I was looking more closely into Source/platform/blink_platform.gypi

BUG=None
R=jamesr@chromium.org,dglazkov@chromium.org

Review URL: https://codereview.chromium.org/26471009

git-svn-id: svn://svn.chromium.org/blink/trunk@159623 bbb929c8-8fbe-4397-9dbb-9b2b20218538