Add a unique ID to GrOpLists and return it from GrRenderTargetContext::addDrawOp

Add a unique ID to GrOpLists and return it from GrRenderTargetContext::addDrawOp

This is to support the preFlush callbacks

Change-Id: I8513ea08b6516681566eceafa789b2ee7925ebce
Reviewed-on: https://skia-review.googlesource.com/9199
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>