Fix font instantiation assertion failure on session restore

It seems in certain cases font fallback returns a fontconfig interface
id which can then later not be reused by Skia. This patch fixes an
incorrect fallback to using the family in that case and returns a nullptr
instead, which fixes the assertion failure. However, we need to
investigate why the Skia typeface creation with the returned
fciId is unsuccessful, filed crbug.com/403348 for this remaining question.

BUG=273261
R=eae,eseidel

Review URL: https://codereview.chromium.org/470493003

git-svn-id: svn://svn.chromium.org/blink/trunk@180214 bbb929c8-8fbe-4397-9dbb-9b2b20218538