Move the WTF_SpinLock.Torture test to webkit_unit_tests

webkit_unit_tests is the lowest-level target that has a WebKit::Platform
which is required for creating WebThreads

BUG=301514
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/61743002

git-svn-id: svn://svn.chromium.org/blink/trunk@161475 bbb929c8-8fbe-4397-9dbb-9b2b20218538