Avoid system fonts when fuzzing

This should make reproducing certain fuzzes easier between oss-fuzz
and a typical dev's desktop.

This was the most straight-forward way I could think of to
accomplish this.  An ideal solution would "compile" a set of
fonts that was not the test set and embed it, but I lack the
domain knowledge to craft such a set.

If this method works ok, we can explore making the font set
more robust and varied.

Bug: 818769
Change-Id: I03eb2bc316caf7aec3ffa88e59ff29d76c8557ec
Reviewed-on: https://skia-review.googlesource.com/c/177800
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
8 files changed