CSSPaint: Move layout tests in csspaint/ to http/tests/csspaint/

third_party/WebKit/LayoutTests/csspaint/resources/test-runner-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:
-//   importPaintWorkletAndTerminateTestAfterAsyncPaint('/* worklet code goes here. */');
-
-function importPaintWorkletAndTerminateTestAfterAsyncPaint(code) {
-    if (window.testRunner) {
-      testRunner.waitUntilDone();
-    }
-
-    var blob = new Blob([code], {type: 'text/javascript'});
-    paintWorklet.addModule(URL.createObjectURL(blob)).then(function() {
-        runAfterLayoutAndPaint(function() {
-            if (window.testRunner) {
-                testRunner.notifyDone();
-            }
-        });
-    });
-}