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

third_party/WebKit/LayoutTests/imported/wpt/offscreen-canvas/the-offscreen-canvas/initial.colour.worker.js

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:initial.colour
-// Description:Initial state is transparent black
-// Note:<p class="notes">Output should be transparent black (not transparent anything-else), but manual
-verification can only confirm that it's transparent - it's not possible to make
-the actual blackness visible.
-
-
-importScripts("/resources/testharness.js");
-importScripts("/common/canvas-tests.js");
-
-var t = async_test("Initial state is transparent black");
-t.step(function() {
-
-var offscreenCanvas = new OffscreenCanvas(100, 50);
-var ctx = offscreenCanvas.getContext('2d');
-
-_assertPixel(offscreenCanvas, 20,20, 0,0,0,0, "20,20", "0,0,0,0");
-
-t.done();
-
-});
-done();