Add SkUNREACHABLE to definition of SK_ABORT.

Remove return statements immediately following SK_ABORTs.

Change-Id: I6446b6ba3a6fb272d84ea27f38cf366095cf801d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/233565
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
74 files changed