Add XferProcessor factory in GrPaint and GrDrawState.

include/gpu/GrBackendProcessorFactory.h

Index: include/gpu/GrBackendProcessorFactory.h
diff --git a/include/gpu/GrBackendProcessorFactory.h b/include/gpu/GrBackendProcessorFactory.h
index 3e4f13314756734cfb5c7957a1db0f969e9da2e9..f62c503005bf63faf8448f82709bbb1bcb5edcad 100644
--- a/include/gpu/GrBackendProcessorFactory.h
+++ b/include/gpu/GrBackendProcessorFactory.h
@@ -140,8 +140,10 @@ private:
 
 class GrFragmentProcessor;
 class GrGeometryProcessor;
+class GrXferProcessor;
 class GrGLFragmentProcessor;
 class GrGLGeometryProcessor;
+class GrGLXferProcessor;
 
 /**
  * Backend processor factory cannot actually create anything, it is up to subclasses to implement
@@ -157,6 +159,15 @@ public:
     virtual GrGLFragmentProcessor* createGLInstance(const GrFragmentProcessor&) const = 0;
 };
 
+class GrBackendXferProcessorFactory : public GrBackendProcessorFactory {
+public:
+    /**
+     * Creates a GrGLProcessor instance that is used both to generate code for the GrProcessor in a
+     * GLSL program and to manage updating uniforms for the program when it is used.
+     */
+    virtual GrGLXferProcessor* createGLInstance(const GrXferProcessor&) const = 0;
+};
+
 class GrBackendGeometryProcessorFactory : public GrBackendProcessorFactory {
 public:
     /**