Remove unnecessary null checking in NavigatorContentUtils

Some functions have checked if document is null. However, document is not null when frame is existed.

BUG=none

Review URL: https://codereview.chromium.org/224733013

git-svn-id: svn://svn.chromium.org/blink/trunk@171928 bbb929c8-8fbe-4397-9dbb-9b2b20218538