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

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

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:2d.imageData.object.set
-// Description:ImageData.data can be modified
-// Note:
-
-importScripts("/resources/testharness.js");
-importScripts("/common/canvas-tests.js");
-
-var t = async_test("ImageData.data can be modified");
-t.step(function() {
-
-var offscreenCanvas = new OffscreenCanvas(100, 50);
-var ctx = offscreenCanvas.getContext('2d');
-
-var imgdata = ctx.getImageData(0, 0, 10, 10);
-imgdata.data[0] = 100;
-_assertSame(imgdata.data[0], 100, "imgdata.data[\""+(0)+"\"]", "100");
-imgdata.data[0] = 200;
-_assertSame(imgdata.data[0], 200, "imgdata.data[\""+(0)+"\"]", "200");
-
-t.done();
-
-});
-done();