Update ConstColor FP to support an input FP.

In the "ignore input" mode, the input FP contributes nothing and is
never sampled. In the "modulate input" cases, the input FP is sampled
as one would expect.

Change-Id: I96717d63d8e3d7ef6aa4eaaf88154c6e5ce47e55
Bug: skia:10217
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/296299
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
diff --git a/src/gpu/GrFragmentProcessor.cpp b/src/gpu/GrFragmentProcessor.cpp
index 999f9fd..435391b 100644
--- a/src/gpu/GrFragmentProcessor.cpp
+++ b/src/gpu/GrFragmentProcessor.cpp
@@ -420,8 +420,8 @@
     SkPMColor4f knownColor;
     int leadingFPsToEliminate = info.initialProcessorsToEliminate(&knownColor);
     if (leadingFPsToEliminate) {
-        std::unique_ptr<GrFragmentProcessor> colorFP(
-                GrConstColorProcessor::Make(knownColor, GrConstColorProcessor::InputMode::kIgnore));
+        std::unique_ptr<GrFragmentProcessor> colorFP = GrConstColorProcessor::Make(
+            /*inputFP=*/nullptr, knownColor, GrConstColorProcessor::InputMode::kIgnore);
         if (leadingFPsToEliminate == cnt) {
             return colorFP;
         }