Oilpan: move WorkerThreadStartupData to the oilpan heap.

R=vegorov@chromium.org,tkent@chromium.org
BUG=340522

Review URL: https://codereview.chromium.org/191003010

git-svn-id: svn://svn.chromium.org/blink/trunk@168903 bbb929c8-8fbe-4397-9dbb-9b2b20218538