Fix initialization of assert scopes.

Fix initialization of assert scopes.

The thread local key for assert scopes can be lazily initialized and
should be independent of the Isolate initialization. Also cleanup the
assert-scope.{cc,h} implementation while I was at it.

R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/609253002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24275 ce2b1a6d-e550-0410-aec6-3dcde31c8c00