Add an assertion and use get() in WeakPtr::operator->.
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@165711 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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