Change WokerThread to use a blink::WebThread

This is a reland of https://codereview.chromium.org/400153002/ in which I forgot the remove the changes made to WebThread for shared timer. The first patch set is equivalent to the previous CL.

BUG=301515

Review URL: https://codereview.chromium.org/423303004

git-svn-id: svn://svn.chromium.org/blink/trunk@179217 bbb929c8-8fbe-4397-9dbb-9b2b20218538