Move setSampleMatrix and setSampledWithExplicitCoords into child registration

Bug: skia:10396
Change-Id: I0c117ab4d95737b76dec5bce16103b9058218fb8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/297065
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
diff --git a/gm/sample_matrix_variable.cpp b/gm/sample_matrix_variable.cpp
index 63abd2b..185f654 100644
--- a/gm/sample_matrix_variable.cpp
+++ b/gm/sample_matrix_variable.cpp
@@ -25,8 +25,7 @@
             : INHERITED(CLASS_ID, kNone_OptimizationFlags)
             , fXOffset(xOffset)
             , fYOffset(yOffset) {
-        child->setSampleMatrix(SkSL::SampleMatrix(SkSL::SampleMatrix::Kind::kVariable));
-        this->registerChildProcessor(std::move(child));
+        this->registerChild(std::move(child), SkSL::SampleMatrix::MakeVariable());
     }
 
     const char* name() const override { return "SampleMatrixVariableEffect"; }