Use sk_sp with GrXferProcessor and use a static factory to handle the nullptr==srcover case.

Bug: skia:
Change-Id: I9c07069fc2aae24fc2884c18939e3a649afc27e7
Reviewed-on: https://skia-review.googlesource.com/11282
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
11 files changed