DevTools: fix font inspection crash

It turns out that FontPlatformDataHarfbuzz doesn't always have
SkTypeface instance; in this case some default system font simply used.
It looks like there's no easy way to figure out the name of default
system font, so we simply fast-return in this case.

BUG=326582

Review URL: https://codereview.chromium.org/111613004

git-svn-id: svn://svn.chromium.org/blink/trunk@163692 bbb929c8-8fbe-4397-9dbb-9b2b20218538