Fix fallback font instantiation for non-sandboxed mode

The fix for font fallback instantiation in r176765 works inside
the sandbox, however this broke it for the case where the sandbox
is inactive.

BUG=285303
R=eae,eseidel

Review URL: https://codereview.chromium.org/341303006

git-svn-id: svn://svn.chromium.org/blink/trunk@176977 bbb929c8-8fbe-4397-9dbb-9b2b20218538