Introduce SkStrikeSpecStorage.

SkStrikeSpecStorage is the centralized class for creating different sets of
SkScalerContextRecs/Effects for different text rendering methods.

It is initailly called SkStrikeSpecStorage, but as a last stage will be
renamed to SkStrikeSpec as it encompasses the current functionality of
the existing SkStrikeSpec.

I'm breaking up a much larger prototype for this CL and several
following CLs. The prototype is at:

https://skia-review.googlesource.com/c/skia/+/209109

Change-Id: I617eaae6fcb4b0b29914c3f1a82c52c81d81aabe
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/212733
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
6 files changed