Add missing null checks in FrameView::setScrollPosition

FrameView::setScrollPosition can get called when there is no
documentElement or when the documentElement has no renderer,
but http://crrev.com/384973003 neglected to add null checks
for these situations. This CL adds the missing null checks.

BUG=399018

Review URL: https://codereview.chromium.org/428953005

git-svn-id: svn://svn.chromium.org/blink/trunk@179614 bbb929c8-8fbe-4397-9dbb-9b2b20218538