Make unittest_config a dependency of webkit_unit_tests.

Prior to this change the webkit_unit_tests target did not enable the plugin for its compilation units.

TBR=eseidel@chromium.org
BUG=

Review URL: https://codereview.chromium.org/301543005

git-svn-id: svn://svn.chromium.org/blink/trunk@174972 bbb929c8-8fbe-4397-9dbb-9b2b20218538