Make StencilPathArgs hold a GrRenderTargetProxy (rather than a GrRenderTarget)

Again, this will be useful when GrSurface does not have a origin field.

Change-Id: I0c4ef5b441eaf38f3489e8631aced11b11fdb6ac
Reviewed-on: https://skia-review.googlesource.com/27540
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
3 files changed