Make sure we never pass a null Frame to Document::canNavigate()

Make sure we never pass a null Frame to Document::canNavigate() and update it
to take the Frame by reference to const. It makes little sense to pass null to
a canNavigate() function, especially considering the implementation was
returning true in this case (there was a FIXME about it).

R=adamk@chromium.org, eseidel@chromium.org

Review URL: https://codereview.chromium.org/307223002

git-svn-id: svn://svn.chromium.org/blink/trunk@175295 bbb929c8-8fbe-4397-9dbb-9b2b20218538