Hook up CSSPaintValue::image to CSS Paint API callback.

third_party/WebKit/LayoutTests/csspaint/resources/test-runner-simple-paint-worklet.js

+// Given a piece of 'code', runs it in the worklet, then once loaded waits for
+// layout and paint, before finishing the test.
+//
+// Usage:
+//   testRunnerSimplePaintWorklet('/* worklet code goes here. */');
+
+function testRunnerSimplePaintWorklet(code) {
+    if (window.testRunner) {
+      testRunner.waitUntilDone();
+    }
+
+    var blob = new Blob([code]);
+    paintWorklet.import(URL.createObjectURL(blob)).then(function() {
+        runAfterLayoutAndPaint(function() {
+            if (window.testRunner) {
+                testRunner.notifyDone();

[Justin Novosad, 2016/04/06 15:52:45]

I am a bit concerned about this call to notifyDone() because it could  interfere
with tests that want trigger more async work after the worklet has run. Maybe
you could just rename the function to make it clearer that it has async
termination behavior. Something like
"importPaintWorkletAndTerminateTestAfterAsyncPaint", or something shorter that
means the same thing.

[ikilpatrick, 2016/04/07 23:03:00]

I couldn't think of a shorter name :) done.

+            }
+        });
+    });
+}