Update content_webkit_unit_test_support dependency

This is a followup to https://codereview.chromium.org/287043002/ where
content_webkit_unit_test_support got collapsed into
test_support_content_target.  This is the only include of that target.

BUG=none

Review URL: https://codereview.chromium.org/286213004

git-svn-id: svn://svn.chromium.org/blink/trunk@174906 bbb929c8-8fbe-4397-9dbb-9b2b20218538