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

third_party/WebKit/LayoutTests/http/tests/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();
             }
         });
     });
 }