Remove a bogus assert in Document::setTitle().

m_titleElement can be either of HTMLTitleElement or SVGTitleElement. The
assertion was wrong.

BUG=301541
TEST=automated.

Review URL: https://codereview.chromium.org/25464002

git-svn-id: svn://svn.chromium.org/blink/trunk@158620 bbb929c8-8fbe-4397-9dbb-9b2b20218538