Add serialized slug VIA to DM for testing

Introduce a new VIA which uses a specialized SkTestCanvas which
has a onDrawGlyphRunList that produces a slug, serializes it,
de-serializes it, and draws it.

Change-Id: Ic612742d1b829f4c351ef07f7a731e440f6d0786
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/611042
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
7 files changed