Move willUseInputColor check to computeInvariantOutput

src/gpu/effects/GrConvexPolyEffect.cpp

Index: src/gpu/effects/GrConvexPolyEffect.cpp
diff --git a/src/gpu/effects/GrConvexPolyEffect.cpp b/src/gpu/effects/GrConvexPolyEffect.cpp
index bafa64d13ae08ed88672c39c2c4c26829aafa011..c84b54085000843877f3eb1a329a51a925ae32f2 100644
--- a/src/gpu/effects/GrConvexPolyEffect.cpp
+++ b/src/gpu/effects/GrConvexPolyEffect.cpp
@@ -46,7 +46,7 @@ private:
     virtual void onComputeInvariantOutput(InvariantOutput* inout) const SK_OVERRIDE {
         if (fRect.isEmpty()) {
             // An empty rect will have no coverage anywhere.
-            inout->setToTransparentBlack();
+            inout->mulByKnownAlpha(0);
         } else {
             inout->mulByUnknownAlpha();
         }