Handle null return value for getLastResortFallbackFont

getNonRetainedLastResortFallbackFont can, despite the name which seems
to suggest otherwise, return 0. Handle this case by bailing out and
asserting.

BUG=341628
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/150103014

git-svn-id: svn://svn.chromium.org/blink/trunk@166678 bbb929c8-8fbe-4397-9dbb-9b2b20218538