Reland r17877 - Introduce a v8::Platform class that bundles embedder callbacks

Over the initial commit, this CL moves the lifetime management of the default
platform to v8.cc from api.cc

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

Review URL: https://codereview.chromium.org/78453003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17970 ce2b1a6d-e550-0410-aec6-3dcde31c8c00