Change mechanism that lets GrRenderTargetContext caller get op list ID.

Previously we relied on GrRenderTargetOpList returning an ID if and only
if it did not merge the op. Upcoming changes to GrRenderTargetOpList will
make it harder to track whether the op was merged.

Only CCPR (and related unit test) requires the op list ID. Now CCPR passes
a callback that is called when the op list is deterimined but before
we know if the op merges or not.

Change-Id: Iead1c3eff8de5a4183e330dd7b57362cbffe0ebb
Reviewed-on: https://skia-review.googlesource.com/c/166624
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Chris Dalton <csmartdalton@google.com>
10 files changed