GN fixes to make webkit_unit_tests link on android

Linux sometimes means android too.

R=brettw@chromium.org

Review URL: https://codereview.chromium.org/556143002

git-svn-id: svn://svn.chromium.org/blink/trunk@181706 bbb929c8-8fbe-4397-9dbb-9b2b20218538