Add push constant support to GrCaps.

Change-Id: I2ef63443a68f3ebc31c7f9ad1e614b320be3e673
Bug: skia:5039
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/324121
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
2 files changed