Add class GrProcessorSet which represents color and coverage FPs and a XPFactory.

Eventually ops can use this to hold their ops and create GrPipelines at flush time.

For now it is used by GrPipelineBuilder.

Change-Id: I0db3892032f2d07238e4c847a790678b3aab456f
Reviewed-on: https://skia-review.googlesource.com/7132
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>