Add an assertion and use get() in WeakPtr::operator->.

This CL makes WTF::WeakPtr<>::operator-> synchronized with
base::WeakPtr::operator-> in Chromium.

BUG=none
TEST=none; no behavior changes.

Review URL: https://codereview.chromium.org/135473003

git-svn-id: svn://svn.chromium.org/blink/trunk@165711 bbb929c8-8fbe-4397-9dbb-9b2b20218538