Delete WTF::yield() and use Platform::yieldCurrentThread instead

BUG=301514
R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/602603002

git-svn-id: svn://svn.chromium.org/blink/trunk@182630 bbb929c8-8fbe-4397-9dbb-9b2b20218538