Remove bogus ASSERT(frame) from RenderObject::willBeDestroyed

This ASSERT was added way back in http://trac.webkit.org/changeset/48070 but
is false in practice (see attached bug). Given that we already handle frame-nullity
in this method, I don't see a reason to carry this legacy ASSERT around with us.

R=eae@chromium.org
BUG=308786

Review URL: https://codereview.chromium.org/35153004

git-svn-id: svn://svn.chromium.org/blink/trunk@160247 bbb929c8-8fbe-4397-9dbb-9b2b20218538