Reland "Remove IsInitialized checks from inlined API functions."

The initialization sequence in Blink has been adapted to use explicit
isolate initialization instead of through calling v8::Null() as the
first API call.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/17642011

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15311 ce2b1a6d-e550-0410-aec6-3dcde31c8c00