RELEASE_ASSERT if last resort fallback font cannot be loaded.

This will help confirm the suspicion that a number of ClusterFuzz reports
from constructTextRun are a result of this failing. There's probably nothing
useful that can be done with a font that lacks any font data anyhow.

BUG=421370

Review URL: https://codereview.chromium.org/641763003

git-svn-id: svn://svn.chromium.org/blink/trunk@183470 bbb929c8-8fbe-4397-9dbb-9b2b20218538