Add XferProcessor factory in GrPaint and GrDrawState.

src/core/SkXfermode.cpp

Index: src/core/SkXfermode.cpp
diff --git a/src/core/SkXfermode.cpp b/src/core/SkXfermode.cpp
index 77c668457b9d4e4b524b94c4909607354ce883fc..5fae9f12564d959cf8b939b31aeb4d97ac4efb23 100644
--- a/src/core/SkXfermode.cpp
+++ b/src/core/SkXfermode.cpp
@@ -6,7 +6,7 @@
  * found in the LICENSE file.
  */
 
-
+#include "GrXferProcessor.h"
 #include "SkXfermode.h"
 #include "SkXfermode_opts_SSE2.h"
 #include "SkXfermode_proccoeff.h"
@@ -680,12 +680,21 @@ bool SkXfermode::asFragmentProcessor(GrFragmentProcessor**, GrTexture*) const {
     return false;
 }
 
-bool SkXfermode::asFragmentProcessorOrCoeff(SkXfermode* xfermode, GrFragmentProcessor** fp,
-                                            Coeff* src, Coeff* dst, GrTexture* background) {
+bool SkXfermode::asXPFactory(GrXPFactory**) const {
+    return false;
+}
+
+bool SkXfermode::asFragmentProcessorOrXPFactoryOrCoeff(SkXfermode* xfermode,
+                                                       GrFragmentProcessor** fp,
+                                                       GrXPFactory** xpf,
+                                                       Coeff* src, Coeff* dst,
+                                                       GrTexture* background) {
     if (NULL == xfermode) {
         return ModeAsCoeff(kSrcOver_Mode, src, dst);
     } else if (xfermode->asCoeff(src, dst)) {
         return true;
+    } else if (NULL == background && xfermode->asXPFactory(xpf)) {
+        return true;
     } else {
         return xfermode->asFragmentProcessor(fp, background);
     }