Make GrOp base class responsible for merging bounds.

Change-Id: If85c00554f46c9601ca3fc3ef95bbf7c5bc81dc0
Reviewed-on: https://skia-review.googlesource.com/c/164686
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
22 files changed