Fix for the FIXME in testing.gyp

DumpRenderTree is gone since a log time now. Merge this resource target
where it is used in web_tests.gyp.

BUG=None
TEST=webkit_unit_tests
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/97933002

git-svn-id: svn://svn.chromium.org/blink/trunk@162951 bbb929c8-8fbe-4397-9dbb-9b2b20218538