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

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

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:2d.imageData.object.readonly
-// Description:ImageData objects properties are read-only
-// Note:
-
-importScripts("/resources/testharness.js");
-importScripts("/common/canvas-tests.js");
-
-var t = async_test("ImageData objects properties are read-only");
-t.step(function() {
-
-var offscreenCanvas = new OffscreenCanvas(100, 50);
-var ctx = offscreenCanvas.getContext('2d');
-
-var imgdata = ctx.getImageData(0, 0, 10, 10);
-var w = imgdata.width;
-var h = imgdata.height;
-var d = imgdata.data;
-imgdata.width = 123;
-imgdata.height = 123;
-imgdata.data = [100,100,100,100];
-_assertSame(imgdata.width, w, "imgdata.width", "w");
-_assertSame(imgdata.height, h, "imgdata.height", "h");
-_assertSame(imgdata.data, d, "imgdata.data", "d");
-_assertSame(imgdata.data[0], 0, "imgdata.data[\""+(0)+"\"]", "0");
-_assertSame(imgdata.data[1], 0, "imgdata.data[\""+(1)+"\"]", "0");
-_assertSame(imgdata.data[2], 0, "imgdata.data[\""+(2)+"\"]", "0");
-_assertSame(imgdata.data[3], 0, "imgdata.data[\""+(3)+"\"]", "0");
-
-t.done();
-
-});
-done();