Add support for multiple Workers in d8

Add d8 API for spawning function on a new thread (Third 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
LOG=n

Committed: https://crrev.com/001ee86e323a722fd4c0761d0b1a2c3a0c8f8e4b
Cr-Commit-Position: refs/heads/master@{#29195}

[binji, 2015-06-19 17:57:51]

binji@chromium.org changed reviewers:
+ adamk@chromium.org

[binji, 2015-06-19 17:57:52]

PTAL, this will fix the tree (at least my part of the failure :)

[binji, 2015-06-19 18:41:25]

Patchset #1 (id:1) has been deleted

[binji, 2015-06-19 18:43:11]

binji@chromium.org changed reviewers:
+ jochen@chromium.org

[binji, 2015-06-19 18:43:12]

This new CL has the previous changes as patch #1, and the new changes with
multiple workers as patch #2.

[jochen (gone - plz use gerrit), 2015-06-22 08:36:08]

lgtm

[binji, 2015-06-22 16:46:42]

The CQ bit was checked by binji@chromium.org

[commit-bot: I haz the power, 2015-06-22 16:46:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1192923002/40001

[commit-bot: I haz the power, 2015-06-22 17:12:35]

Committed patchset #2 (id:40001)

[commit-bot: I haz the power, 2015-06-22 17:12:50]

Patchset 2 (id:??) landed as
https://crrev.com/001ee86e323a722fd4c0761d0b1a2c3a0c8f8e4b
Cr-Commit-Position: refs/heads/master@{#29195}