Add offscreen-canvas to the list of wpt directories to skip.

third_party/WebKit/LayoutTests/imported/wpt/offscreen-canvas/pixel-manipulation/2d.imageData.create2.negative.worker.js

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:2d.imageData.create2.negative
-// Description:createImageData(sw, sh) takes the absolute magnitude of the size arguments
-// Note:
-
-importScripts("/resources/testharness.js");
-importScripts("/common/canvas-tests.js");
-
-var t = async_test("createImageData(sw, sh) takes the absolute magnitude of the size arguments");
-t.step(function() {
-
-var offscreenCanvas = new OffscreenCanvas(100, 50);
-var ctx = offscreenCanvas.getContext('2d');
-
-var imgdata1 = ctx.createImageData(10, 20);
-var imgdata2 = ctx.createImageData(-10, 20);
-var imgdata3 = ctx.createImageData(10, -20);
-var imgdata4 = ctx.createImageData(-10, -20);
-_assertSame(imgdata1.data.length, imgdata2.data.length, "imgdata1.data.length", "imgdata2.data.length");
-_assertSame(imgdata2.data.length, imgdata3.data.length, "imgdata2.data.length", "imgdata3.data.length");
-_assertSame(imgdata3.data.length, imgdata4.data.length, "imgdata3.data.length", "imgdata4.data.length");
-
-t.done();
-
-});
-done();