Replace manually-implemented lerp with mix().

Change-Id: I6bf3cac6df654a0b23ced250df90716f6856ed29
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/307197
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>

src/gpu/gradients/GrSingleIntervalGradientColorizer.fp

@@ -5,7 +5,7 @@
  * found in the LICENSE file.
  */
 
-// This only supports a 2-color single interval so it is a simple  linear interpolation between the
+// This only supports a 2-color single interval so it is a simple linear interpolation between the
 // two end points based on t. But it serves as a good test for connecting all of the plumbing into a
 // functional gradient shader.
 
@@ -17,5 +17,5 @@ void main() {
 
     // Clamping and/or wrapping was already handled by the parent shader so the output color is a
     // simple lerp.
-    sk_OutColor = (1 - t) * start + t * end;
+    sk_OutColor = mix(start, end, t);
 }

src/gpu/gradients/generated/GrSingleIntervalGradientColorizer.cpp

@@ -35,7 +35,7 @@ public:
                                                   "end");
         fragBuilder->codeAppendf(
                 R"SkSL(half t = %s.x;
-%s = (1.0 - t) * %s + t * %s;
+%s = mix(%s, %s, t);
 )SkSL",
                 args.fInputColor, args.fOutputColor, args.fUniformHandler->getUniformCStr(startVar),
                 args.fUniformHandler->getUniformCStr(endVar));