Remove render target unique ID virtual from GrOp.

Remove render target unique ID virtual from GrOp.

GrRenderTargetOpList now stores the IDs along side each op.

This should put us closer to using proxy IDs and not forcing early render target instantiation as many comments point towards.

Change-Id: I1ee82b01a0818a80d2bcac39fdf3a4ee7dccecc9
Reviewed-on: https://skia-review.googlesource.com/6403
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>