Fix WeakPtr::UnspecifiedBoolType()

WeakPtr::UnspecifiedBoolType() does not take into account value of WeakReference's m_ptr - this should be fixed.

Review URL: https://codereview.chromium.org/25078006

git-svn-id: svn://svn.chromium.org/blink/trunk@158559 bbb929c8-8fbe-4397-9dbb-9b2b20218538