Add XferProcessor factory in GrPaint and GrDrawState.

In this CL the XP should have zero effect on the actual rendering pipeline.

BUG=skia:

Review URL: https://codereview.chromium.org/751283002
diff --git a/src/gpu/GrProcessor.cpp b/src/gpu/GrProcessor.cpp
index d850d68..8aeef04 100644
--- a/src/gpu/GrProcessor.cpp
+++ b/src/gpu/GrProcessor.cpp
@@ -28,6 +28,13 @@
 }
 
 template<>
+SkTArray<GrProcessorTestFactory<GrXferProcessor>*, true>*
+GrProcessorTestFactory<GrXferProcessor>::GetFactories() {
+    static SkTArray<GrProcessorTestFactory<GrXferProcessor>*, true> gFactories;
+    return &gFactories;
+}
+
+template<>
 SkTArray<GrProcessorTestFactory<GrGeometryProcessor>*, true>*
 GrProcessorTestFactory<GrGeometryProcessor>::GetFactories() {
     static SkTArray<GrProcessorTestFactory<GrGeometryProcessor>*, true> gFactories;
@@ -41,6 +48,7 @@
  */
 static const int kFPFactoryCount = 37;
 static const int kGPFactoryCount = 14;
+static const int kXPFactoryCount = 0;
 
 template<>
 void GrProcessorTestFactory<GrFragmentProcessor>::VerifyFactoryCount() {
@@ -56,6 +64,13 @@
     }
 }
 
+template<>
+void GrProcessorTestFactory<GrXferProcessor>::VerifyFactoryCount() {
+    if (kXPFactoryCount != GetFactories()->count()) {
+        SkFAIL("Wrong number of xfer processor factories!");
+    }
+}
+
 #endif
 
 namespace GrProcessorUnitTest {