Add SkPaintParamsKey::Header structure.

This was originally done with the intent of adding a third field
(number of pointers in the data) but we may be able to get away with
keeping the two-byte Header after all. Having the data in an actual
struct does make refactoring a lot easier, at any rate.

Change-Id: I25a4e46b45f3f9c93eb16bc4d51b6624d7e319ba
Bug: skia:13428
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/550183
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
4 files changed