Added premulFourChannelColor to GrInvariantOutput

include/gpu/GrInvariantOutput.h

Index: include/gpu/GrInvariantOutput.h
diff --git a/include/gpu/GrInvariantOutput.h b/include/gpu/GrInvariantOutput.h
index 8cf4bf59c3244d2552e45b926ade4535061ee1b4..6e2cbe84f9a07dce127be0113836afd35d50f5c4 100644
--- a/include/gpu/GrInvariantOutput.h
+++ b/include/gpu/GrInvariantOutput.h
@@ -197,6 +197,18 @@ public:
         SkDEBUGCODE(this->validate());
     }
 
+    void premulFourChannelColor() {
+        SkDEBUGCODE(this->validate());
+        SkASSERT(!fIsSingleComponent);
+        fNonMulStageFound = true;
+        if (!(fValidFlags & kA_GrColorComponentFlag)) {
+            fValidFlags = kNone_GrColorComponentFlags;
+        } else {
+            fColor = GrPremulColor(fColor);
+        }
+        SkDEBUGCODE(this->validate());
+    }
+
     void invalidateComponents(GrColorComponentFlags invalidateFlags, ReadInput readsInput) {
         SkDEBUGCODE(this->validate());
         fValidFlags = (fValidFlags & ~invalidateFlags);