Introduce SkStrikePromise.

SkStrikePromise is a way to get an SkStrike with information
provided by an SkStrike or a RemoteStrike when the SkStrike
is needed by the GPU code for actual glyph data.

For how this fits the big picture see:
https://skia-review.googlesource.com/c/skia/+/568799

Change-Id: Ie063a329fda11445d0b1c2c02cc3e448d069b84f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/568286
Reviewed-by: Robert Phillips <robertphillips@google.com>
4 files changed