Cleanup: Also simplify wtf_unittests target.

It is seems redundant to have a separate target only for the main() entry.

It looks cleaner to have everything contained into a single target imo.

Previous patch did it for blink_heap_unittests and
blink_platform_unittests in
https://src.chromium.org/viewvc/blink?revision=179353&view=revision

BUG=None
TEST=wtf_unittests
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/436793003

git-svn-id: svn://svn.chromium.org/blink/trunk@179365 bbb929c8-8fbe-4397-9dbb-9b2b20218538