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

third_party/WebKit/LayoutTests/imported/wpt/offscreen-canvas/drawing-rectangles-to-the-canvas/2d.clearRect.globalalpha.worker.js

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:2d.clearRect.globalalpha
-// Description:clearRect is not affected by globalAlpha
-// Note:
-
-importScripts("/resources/testharness.js");
-importScripts("/common/canvas-tests.js");
-
-var t = async_test("clearRect is not affected by globalAlpha");
-t.step(function() {
-
-var offscreenCanvas = new OffscreenCanvas(100, 50);
-var ctx = offscreenCanvas.getContext('2d');
-
-ctx.fillStyle = '#f00';
-ctx.fillRect(0, 0, 100, 50);
-ctx.globalAlpha = 0.1;
-ctx.clearRect(0, 0, 100, 50);
-_assertPixel(offscreenCanvas, 50,25, 0,0,0,0, "50,25", "0,0,0,0");
-
-t.done();
-
-});
-done();