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

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

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:2d.imageData.create2.round
-// Description:createImageData(w, h) is rounded the same as getImageData(0, 0, w, h)
-// Note:
-
-importScripts("/resources/testharness.js");
-importScripts("/common/canvas-tests.js");
-
-var t = async_test("createImageData(w, h) is rounded the same as getImageData(0, 0, w, h)");
-t.step(function() {
-
-var offscreenCanvas = new OffscreenCanvas(100, 50);
-var ctx = offscreenCanvas.getContext('2d');
-
-var imgdata1 = ctx.createImageData(10.01, 10.99);
-var imgdata2 = ctx.getImageData(0, 0, 10.01, 10.99);
-_assertSame(imgdata1.width, imgdata2.width, "imgdata1.width", "imgdata2.width");
-_assertSame(imgdata1.height, imgdata2.height, "imgdata1.height", "imgdata2.height");
-
-t.done();
-
-});
-done();