Change WokerThread to use a blink::WebThread

This is a reland of https://codereview.chromium.org/423303004/, which was reverted due to memory leak.

BUG=301515

Review URL: https://codereview.chromium.org/438523002

git-svn-id: svn://svn.chromium.org/blink/trunk@179309 bbb929c8-8fbe-4397-9dbb-9b2b20218538