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

third_party/WebKit/LayoutTests/imported/wpt/offscreen-canvas/the-offscreen-canvas/2d.getcontext.exists.worker.js

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:2d.getcontext.exists
-// Description:The 2D context is implemented
-// Note:
-
-importScripts("/resources/testharness.js");
-importScripts("/common/canvas-tests.js");
-
-var t = async_test("The 2D context is implemented");
-t.step(function() {
-
-var offscreenCanvas = new OffscreenCanvas(100, 50);
-var ctx = offscreenCanvas.getContext('2d');
-
-var offscreenCanvas2 = new OffscreenCanvas(100, 50);
-_assertDifferent(offscreenCanvas2.getContext('2d'), null, "offscreenCanvas2.getContext('2d')", "null");
-
-t.done();
-
-});
-done();