Add some backend flexibility for shader declarations in ProgramBuilding.

src/gpu/gl/GrGLVaryingHandler.h

Index: src/gpu/gl/GrGLVaryingHandler.h
diff --git a/src/gpu/gl/GrGLVaryingHandler.h b/src/gpu/gl/GrGLVaryingHandler.h
index 50a87adc033017dfa36bdbaeeeac5edf0469cd8f..fe8c3dca69646ae596712574bfdfc2fb3eb0a825 100644
--- a/src/gpu/gl/GrGLVaryingHandler.h
+++ b/src/gpu/gl/GrGLVaryingHandler.h
@@ -24,6 +24,8 @@ public:
                                            GrSLPrecision fsPrecision = kDefault_GrSLPrecision);
 
 private:
+    void onFinalize() override {}
+
     typedef GrGLProgramDataManager::VaryingInfo VaryingInfo;
     typedef GrGLProgramDataManager::VaryingInfoArray VaryingInfoArray;