Push usage of GrColor4f into OverrideInput

Just a change to preserve precision.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2324553002

Review-Url: https://codereview.chromium.org/2324553002
diff --git a/include/gpu/GrFragmentProcessor.h b/include/gpu/GrFragmentProcessor.h
index ffbf576..f955207 100644
--- a/include/gpu/GrFragmentProcessor.h
+++ b/include/gpu/GrFragmentProcessor.h
@@ -45,7 +45,7 @@
      *  The parent will ignore its input color and instead feed the passed in color as input to the
      *  child.
      */
-    static sk_sp<GrFragmentProcessor> OverrideInput(sk_sp<GrFragmentProcessor>, GrColor);
+    static sk_sp<GrFragmentProcessor> OverrideInput(sk_sp<GrFragmentProcessor>, GrColor4f);
 
     /**
      *  Returns a fragment processor that premuls the input before calling the passed in fragment