Revert 17877 - Introduce a v8::Platform class that bundles embedder callbacks

This also reverts 17879 and 17880.

BUG=v8:3015
TBR=svenpanne@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/76323002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17881 ce2b1a6d-e550-0410-aec6-3dcde31c8c00