Determine whether we can tweakAlphaForCoverage during Pipeline/XP creation.

BUG=skia:

Review URL: https://codereview.chromium.org/927623002
diff --git a/src/gpu/GrPrimitiveProcessor.h b/src/gpu/GrPrimitiveProcessor.h
index a088d38..45668e9 100644
--- a/src/gpu/GrPrimitiveProcessor.h
+++ b/src/gpu/GrPrimitiveProcessor.h
@@ -83,6 +83,7 @@
     bool fCoverageIgnored;
     GrColor fOverrideColor;
     bool fUsesLocalCoords;
+    bool fCanTweakAlphaForCoverage;
 };
 
 /*