Introduce a v8::Platform class that bundles embedder callbacks

Introduce a v8::Platform class that bundles embedder callbacks

Also provide a default implementation to use in cctests.

For now, there are just two thread releated callbacks. In future CLs, I will
move callbacks registered e.g. via V8::SetFooCallback over.

BUG=v8:3015
R=svenpanne@chromium.org, danno@chromium.org
LOG=no

Review URL: https://codereview.chromium.org/75283002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17877 ce2b1a6d-e550-0410-aec6-3dcde31c8c00