Add d8 API for spawning function on a new thread (Second try)

Add d8 API for spawning function on a new thread (Second try)

This API closely matches the Worker API. The differences:

1) The argument to the Worker constructor is a function to run, not a script.
2) Receiving a message from a worker is a synchronous API (as there is no event
loop).

The serialization done here is not robust as the real DOM implementation. For
example, recursive data structures or otherwise duplicated objects are not
allowed.

BUG=chromium:497295
R=jochen@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1185643004

Cr-Commit-Position: refs/heads/master@{#29126}

Review URL: https://codereview.chromium.org/1195613003

Cr-Commit-Position: refs/heads/master@{#29158}