[platform] Implement a worker pool

[platform] Implement a worker pool

We don't use the worker pool yet, however, there are tests. Yay. The
next step is to use the worker pool for parallel sweeping.

I've also started to move the platform related files into a sub
directory. The goal is to eventually build all the platform stuff as
a separate library which is used by d8 and cctest (and other embedders
that wish to use the default implementation) but not by chromium.

BUG=v8:3015
R=hpayer@chromium.org, svenpanne@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/104583003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@18380 ce2b1a6d-e550-0410-aec6-3dcde31c8c00