Revert "Remove IsInitialized checks from inlined API functions."

This reverts r15277 due to failures in layout tests. Apparently Blink
still initializes the Isolate by calling v8::Null() as the first API
function on some paths.

TBR=svenpanne@chromium.org
TEST=webkit:crypto/worker-random-values-concurrent.html

Review URL: https://codereview.chromium.org/17577008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15281 ce2b1a6d-e550-0410-aec6-3dcde31c8c00