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

third_party/WebKit/LayoutTests/imported/wpt/offscreen-canvas/pixel-manipulation/2d.imageData.put.alpha.worker.js

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:2d.imageData.put.alpha
-// Description:putImageData() puts non-solid image data correctly
-// Note:
-
-importScripts("/resources/testharness.js");
-importScripts("/common/canvas-tests.js");
-
-var t = async_test("putImageData() puts non-solid image data correctly");
-t.step(function() {
-
-var offscreenCanvas = new OffscreenCanvas(100, 50);
-var ctx = offscreenCanvas.getContext('2d');
-
-ctx.fillStyle = 'rgba(0, 255, 0, 0.25)';
-ctx.fillRect(0, 0, 100, 50)
-var imgdata = ctx.getImageData(0, 0, 100, 50);
-ctx.fillStyle = '#f00';
-ctx.fillRect(0, 0, 100, 50)
-ctx.putImageData(imgdata, 0, 0);
-_assertPixelApprox(offscreenCanvas, 50,25, 0,255,0,64, "50,25", "0,255,0,64", 2);
-
-t.done();
-
-});
-done();