Test that GrFragmentProcessors work without input colors.

src/gpu/effects/GrExtractAlphaFragmentProcessor.cpp

Index: src/gpu/effects/GrExtractAlphaFragmentProcessor.cpp
diff --git a/src/gpu/effects/GrExtractAlphaFragmentProcessor.cpp b/src/gpu/effects/GrExtractAlphaFragmentProcessor.cpp
index c5ee9b8571c285c9b44572fe252c343dbf8fcd8b..8f6af65ca77be81034ab62d8c8f906f5b107a435 100644
--- a/src/gpu/effects/GrExtractAlphaFragmentProcessor.cpp
+++ b/src/gpu/effects/GrExtractAlphaFragmentProcessor.cpp
@@ -14,9 +14,13 @@ public:
     GLExtractAlphaFragmentProcessor() {}
 
     void emitCode(EmitArgs& args) override {
-        GrGLFragmentBuilder* fsBuilder = args.fBuilder->getFragmentShaderBuilder();
-        fsBuilder->codeAppendf("vec4 alpha4 = %s.aaaa;", args.fInputColor);
-        this->emitChild(0, "alpha4", args.fOutputColor, args);
+        if (args.fInputColor) {
+            GrGLFragmentBuilder* fsBuilder = args.fBuilder->getFragmentShaderBuilder();
+            fsBuilder->codeAppendf("vec4 alpha4 = %s.aaaa;", args.fInputColor);
+            this->emitChild(0, "alpha4", args.fOutputColor, args);
+        } else {
+            this->emitChild(0, nullptr, args.fOutputColor, args);
+        }
     }
 
 private: