Create helper functions to use in computeInvariantOutput calls

src/gpu/GrOvalRenderer.cpp

Index: src/gpu/GrOvalRenderer.cpp
diff --git a/src/gpu/GrOvalRenderer.cpp b/src/gpu/GrOvalRenderer.cpp
index 4dda3b5113c7816a8104bd5d40fea15d1d79cf73..3ba486186b17e45e10d1c3cadece2fb7ff27cd52 100644
--- a/src/gpu/GrOvalRenderer.cpp
+++ b/src/gpu/GrOvalRenderer.cpp
@@ -146,8 +146,7 @@ private:
     }
 
     virtual void onComputeInvariantOutput(InvariantOutput* inout) const SK_OVERRIDE {
-        inout->fValidFlags = 0;
-        inout->fIsSingleComponent = false;
+        inout->mulByUnknownAlpha();
     }
 
     const GrShaderVar& fInCircleEdge;
@@ -287,8 +286,7 @@ private:
     }
 
     virtual void onComputeInvariantOutput(InvariantOutput* inout) const SK_OVERRIDE {
-        inout->fValidFlags = 0;
-        inout->fIsSingleComponent = false;
+        inout->mulByUnknownAlpha();
     }
 
     const GrShaderVar& fInEllipseOffset;
@@ -456,8 +454,7 @@ private:
     }
 
     virtual void onComputeInvariantOutput(InvariantOutput* inout) const SK_OVERRIDE {
-        inout->fValidFlags = 0;
-        inout->fIsSingleComponent = false;
+        inout->mulByUnknownAlpha();
     }
 
     const GrShaderVar& fInEllipseOffsets0;