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.negative.worker.js

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:2d.clearRect.negative
-// Description:clearRect of negative sizes works
-// Note:
-
-importScripts("/resources/testharness.js");
-importScripts("/common/canvas-tests.js");
-
-var t = async_test("clearRect of negative sizes works");
-t.step(function() {
-
-var offscreenCanvas = new OffscreenCanvas(100, 50);
-var ctx = offscreenCanvas.getContext('2d');
-
-ctx.fillStyle = '#f00';
-ctx.fillRect(0, 0, 100, 50);
-ctx.clearRect(0, 0, 50, 25);
-ctx.clearRect(100, 0, -50, 25);
-ctx.clearRect(0, 50, 50, -25);
-ctx.clearRect(100, 50, -50, -25);
-_assertPixel(offscreenCanvas, 25,12, 0,0,0,0, "25,12", "0,0,0,0");
-_assertPixel(offscreenCanvas, 75,12, 0,0,0,0, "75,12", "0,0,0,0");
-_assertPixel(offscreenCanvas, 25,37, 0,0,0,0, "25,37", "0,0,0,0");
-_assertPixel(offscreenCanvas, 75,37, 0,0,0,0, "75,37", "0,0,0,0");
-
-t.done();
-
-});
-done();