[canvaskit] Fix test for drawOnce

I belatedly realized that drawOnce's test would always be blank now
that drawOnce is async.

Change-Id: If30bbdd895039bd8de050a59dd348ba6849c5835
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/266631
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>

Author: kjlubick

modules/canvaskit/CHANGELOG.md

@@ -5,6 +5,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Added
+ - `SkSurface.drawOnce` for drawing a single frame (in addition to already existing
+   `SkSurface.requestAnimationFrame` for animation logic).
 
 ## [0.12.0] - 2020-01-22
 

modules/canvaskit/interface.js

@@ -762,6 +762,8 @@ CanvasKit.onRuntimeInitialized = function() {
     }.bind(this));
   }
 
+  // drawOnce will dispose of the surface after drawing the frame using the provided
+  // callback.
   CanvasKit.SkSurface.prototype.drawOnce = function(callback, dirtyRect) {
     if (!this._cached_canvas) {
       this._cached_canvas = this.getCanvas();

modules/canvaskit/tests/core.spec.js

@@ -389,13 +389,17 @@ describe('Core canvas behavior', function() {
                 path.lineTo(30, 20);
                 path.lineTo(40, 10);
                 canvas.drawPath(path, paint);
-
                 path.delete();
                 paint.delete();
+                // surface hasn't been flushed yet (nor should we call flush
+                // ourselves), so reportSurface would likely be blank if we
+                // were to call it.
+                done();
             }
             surface.drawOnce(drawFrame);
-
-            reportSurface(surface, 'drawOnce', done);
+            // Reminder: drawOnce is async. In this test, we are just making
+            // sure the drawOnce function is there and doesn't crash, so we can
+            // just call done() when the frame is rendered.
         }));
     });