Update clearOp for split-OpList world (take 3)

It would reduce a lot of noise if the GrRenderTargetOpList kept a pointer to the GrCaps but, for now, I'm trying to shrink the GrRTOpList, not expand it.

Reland of: https://skia-review.googlesource.com/c/17323/ (Update clearOp for split-OpList world)

Change-Id: Ia61ce4d6ce245380fc9651928a46c22039fc12cf
Reviewed-on: https://skia-review.googlesource.com/18026
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
12 files changed