Add task related methods to WorkerThread.

This CL adds methods to WorkerThread that are present on WorkerRunLoop in preparation of removing the latter. It is just forwarding the calls for now, but makes it easier to remove the WorkerRunLoop later on.
There is also some cleanup of headers in other files.

BUG=301515

Review URL: https://codereview.chromium.org/405933003

git-svn-id: svn://svn.chromium.org/blink/trunk@178626 bbb929c8-8fbe-4397-9dbb-9b2b20218538