Add CanvasKit bindings for SkRuntimeEffect's uniform data

Written to use the exact same names and semantics as particles.

Cq-Include-Trybots: luci.skia.skia.primary:Test-Debian10-EMCC-GCE-CPU-AVX2-wasm-Release-All-CanvasKit,Test-Debian10-EMCC-GCE-GPU-AVX2-wasm-Release-All-CanvasKit
Change-Id: I4031efbce06527a519f1ce8c261f79231554e1ef
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/365701
Reviewed-by: Joe Gregorio <jcgregorio@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/modules/canvaskit/externs.js b/modules/canvaskit/externs.js
index 3c33ebe..c6d6e84 100644
--- a/modules/canvaskit/externs.js
+++ b/modules/canvaskit/externs.js
@@ -168,6 +168,10 @@
   RuntimeEffect: {
     // public API (from JS bindings)
     Make: function() {},
+    getUniform: function() {},
+    getUniformCount: function() {},
+    getUniformFloatCount: function() {},
+    getUniformName: function() {},
     prototype: {
       makeShader: function() {},
       makeShaderWithChildren: function() {},