Avoid crash in FontFallbackList when LastResortFallbackFont is null

Blink r176963 made FontFallbackList::determinePrimarySimpleFontData crash
when getLastResortFallbackFont() returns null, but callers may be able to
handle a NULL return (see also http://crbug.com/383542#c23).
This patch lets determinePrimarySimpleFontData() propagate NULL returns
to the callers.

BUG=389536,390272,390284

Review URL: https://codereview.chromium.org/390183005

git-svn-id: svn://svn.chromium.org/blink/trunk@178302 bbb929c8-8fbe-4397-9dbb-9b2b20218538