Return an unordered_set of unresolved codepoints

Change-Id: I24d6df800c2b3d00b75b7d045226dc541d9cc24b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/695716
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Julia Lavrova <jlavrova@google.com>
5 files changed