Disable GrOp chaining.

It violates painter's order when merging/chaining an op that is already
in a chain.

Improve OpChainTest so that it would have caught painter's order bug.

Bug: skia:8491
Bug: chromium:894015
Change-Id: Ibfec2d377c903abbb40136e16804137c76d1844c
Reviewed-on: https://skia-review.googlesource.com/c/164609
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
2 files changed