Add helper method to clone and register a child FP.

This will remove common boilerplate from our gen-code, and gives us a
place to put common child-cloning boilerplate.

Change-Id: I6101655af89d4c5844ec908b81ce4f6e5d59f834
Bug: skia:10217
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/296177
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
diff --git a/src/gpu/effects/generated/GrCircleEffect.cpp b/src/gpu/effects/generated/GrCircleEffect.cpp
index 64ee932..b85ad8f 100644
--- a/src/gpu/effects/generated/GrCircleEffect.cpp
+++ b/src/gpu/effects/generated/GrCircleEffect.cpp
@@ -112,11 +112,7 @@
         , center(src.center)
         , radius(src.radius) {
     if (src.inputFP_index >= 0) {
-        auto inputFP_clone = src.childProcessor(src.inputFP_index).clone();
-        if (src.childProcessor(src.inputFP_index).isSampledWithExplicitCoords()) {
-            inputFP_clone->setSampledWithExplicitCoords();
-        }
-        inputFP_index = this->registerChildProcessor(std::move(inputFP_clone));
+        inputFP_index = this->cloneAndRegisterChildProcessor(src.childProcessor(src.inputFP_index));
     }
 }
 std::unique_ptr<GrFragmentProcessor> GrCircleEffect::clone() const {