Now that we have ASSERT() macro in src/base/logging.h we can use it in
safe_conversions_impl.h, re-enable this assertion and fix the TODO
there.

BUG=None
TEST=None
R=jochen@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/369483003

Patch from Thiago Farina <tfarina@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22138 ce2b1a6d-e550-0410-aec6-3dcde31c8c00