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

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

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:2d.imageData.get.nonpremul
-// Description:getImageData() returns non-premultiplied colours
-// Note:
-
-importScripts("/resources/testharness.js");
-importScripts("/common/canvas-tests.js");
-
-var t = async_test("getImageData() returns non-premultiplied colours");
-t.step(function() {
-
-var offscreenCanvas = new OffscreenCanvas(100, 50);
-var ctx = offscreenCanvas.getContext('2d');
-
-ctx.fillStyle = 'rgba(255, 255, 255, 0.5)';
-ctx.fillRect(0, 0, 100, 50);
-var imgdata = ctx.getImageData(10, 10, 10, 10);
-_assert(imgdata.data[0] > 200, "imgdata.data[\""+(0)+"\"] > 200");
-_assert(imgdata.data[1] > 200, "imgdata.data[\""+(1)+"\"] > 200");
-_assert(imgdata.data[2] > 200, "imgdata.data[\""+(2)+"\"] > 200");
-_assert(imgdata.data[3] > 100, "imgdata.data[\""+(3)+"\"] > 100");
-_assert(imgdata.data[3] < 200, "imgdata.data[\""+(3)+"\"] < 200");
-
-t.done();
-
-});
-done();