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

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

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