Add shader makers to SkShader externs

Bug: skia:9843
Change-Id: I98ba2e41563b8b3dcec523ae7a2340b9aacfec2d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/275963
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Nathaniel Nifong <nifong@google.com>

modules/canvaskit/externs.js

@@ -462,6 +462,9 @@ var CanvasKit = {
     Color: function() {},
     Empty: function() {},
     Lerp: function() {},
+    MakeLinearGradient: function() {},
+    MakeRadialGradient: function() {},
+    MakeTwoPointConicalGradient: function() {},
 
     _Blend: function() {},
     _Lerp: function() {},