V8::Initialize() can be called from various threads simultaneously, so
everything should be done only once per process or within the Isolate. For
registering extensions, we do the former now.

R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/15564002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14730 ce2b1a6d-e550-0410-aec6-3dcde31c8c00