Update XPF invariant info to not account for conflation

Renames getInvariantOutput to getInvariantBlendedColor on GrXPFactory
and redefines it to not account for coverage conflation. This is the
information that all the callsites actually wanted to know.

BUG=skia:

Review URL: https://codereview.chromium.org/1161273005
19 files changed