Update GrPaint APIs to reflect lack of multiple color processors.

Change-Id: Ic7799b3c5f4294cba9ff72f8c11a2ad285ab189f
Bug: skia:10217
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/304738
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
diff --git a/src/gpu/GrRenderTargetContext.cpp b/src/gpu/GrRenderTargetContext.cpp
index f439030..560c0d2 100644
--- a/src/gpu/GrRenderTargetContext.cpp
+++ b/src/gpu/GrRenderTargetContext.cpp
@@ -2119,7 +2119,7 @@
     if (!fp) {
         return false;
     }
-    paint.addColorFragmentProcessor(std::move(fp));
+    paint.setColorFragmentProcessor(std::move(fp));
 
     this->fillRectToRect(
             nullptr, std::move(paint), GrAA::kNo, SkMatrix::I(),