Check XpFactory equality in DrawState

BUG=skia:

Review URL: https://codereview.chromium.org/767873006
diff --git a/src/gpu/effects/GrPorterDuffXferProcessor.cpp b/src/gpu/effects/GrPorterDuffXferProcessor.cpp
index 04a7b1f..55e0c93 100644
--- a/src/gpu/effects/GrPorterDuffXferProcessor.cpp
+++ b/src/gpu/effects/GrPorterDuffXferProcessor.cpp
@@ -65,6 +65,11 @@
 
 ///////////////////////////////////////////////////////////////////////////////
 
+GrPorterDuffXPFactory::GrPorterDuffXPFactory(GrBlendCoeff src, GrBlendCoeff dst)
+    : fSrc(src), fDst(dst) {
+    this->initClassID<GrPorterDuffXPFactory>();
+}
+
 GrXPFactory* GrPorterDuffXPFactory::Create(SkXfermode::Mode mode) {
     switch (mode) {
         case SkXfermode::kClear_Mode: {