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