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
diff --git a/include/gpu/effects/GrCoverageSetOpXP.h b/include/gpu/effects/GrCoverageSetOpXP.h
index 2f4530c..6067b5e 100644
--- a/include/gpu/effects/GrCoverageSetOpXP.h
+++ b/include/gpu/effects/GrCoverageSetOpXP.h
@@ -28,8 +28,8 @@
         return true;
     }
 
-    void getInvariantOutput(const GrProcOptInfo& colorPOI, const GrProcOptInfo& coveragePOI,
-                            GrXPFactory::InvariantOutput*) const override;
+    void getInvariantBlendedColor(const GrProcOptInfo& colorPOI,
+                                  GrXPFactory::InvariantBlendedColor*) const override;
 
 private:
     GrCoverageSetOpXPFactory(SkRegion::Op regionOp, bool invertCoverage);