Complete mipmapmode enum

Change-Id: I95d491ae0364897b3f48d81d0ca6fb5714c1ae9e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/388718
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Mike Reed <reed@google.com>
Auto-Submit: Mike Reed <reed@google.com>

modules/canvaskit/externs.js

@@ -435,6 +435,12 @@ var CanvasKit = {
     MakeBlur: function() {},
   },
 
+  MipmapMode: {
+    None: {},
+    Nearest: {},
+    Linear: {},
+  },
+
   Paint: {
     // public API (from C++ bindings)
     /** @return {CanvasKit.Paint} */