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

third_party/WebKit/LayoutTests/imported/wpt/offscreen-canvas/the-offscreen-canvas/context.unrecognised.nullsuffix.worker.js

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:context.unrecognised.nullsuffix
-// Description:Context name "2d" plus a "\0" suffix is unrecognised
-// Note:
-
-importScripts("/resources/testharness.js");
-importScripts("/common/canvas-tests.js");
-
-var t = async_test("Context name \"2d\" plus a \"\\0\" suffix is unrecognised");
-t.step(function() {
-
-var offscreenCanvas = new OffscreenCanvas(100, 50);
-var ctx = offscreenCanvas.getContext('2d');
-
-var offscreenCanvas2 = new OffscreenCanvas(100, 50);
-_assertSame(offscreenCanvas2.getContext("2d\0"), null, "offscreenCanvas2.getContext(\"2d\\0\")", "null");
-
-t.done();
-
-});
-done();