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: https://v8.googlecode.com/svn/branches/bleeding_edge@17877 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Fix push-to-trunk runtime call.

This broke after a recent refactoring.

BUG=
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/76193002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17878 ce2b1a6d-e550-0410-aec6-3dcde31c8c00