Add XP to handle the cases where we disable color write.

BUG=skia:

Review URL: https://codereview.chromium.org/787233003
diff --git a/src/gpu/GrPaint.cpp b/src/gpu/GrPaint.cpp
index 1997796..da6408c 100644
--- a/src/gpu/GrPaint.cpp
+++ b/src/gpu/GrPaint.cpp
@@ -41,7 +41,7 @@
                                         kRGBA_GrColorComponentFlags, false);
 
     GrXPFactory::InvariantOutput output;
-    fXPFactory->getInvariantOutput(colorProcInfo, coverageProcInfo, true, &output);
+    fXPFactory->getInvariantOutput(colorProcInfo, coverageProcInfo, &output);
 
     if (kRGBA_GrColorComponentFlags == output.fBlendedColorFlags &&
         0xFF == GrColorUnpackA(output.fBlendedColor)) {