Add list of dependents to GrOpList class

This is pulled out of: https://skia-review.googlesource.com/c/skia/+/143113 (Reduce arbitrary opList splitting when sorting (take 3)) and is necessary for incremental tological sorting of opLists.

Change-Id: I080cee2c54f5a61dceea67037242f6c6b3e01b8d
Reviewed-on: https://skia-review.googlesource.com/145641
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
2 files changed