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

third_party/WebKit/LayoutTests/imported/wpt/offscreen-canvas/compositing/2d.composite.globalAlpha.invalid.worker.js

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:2d.composite.globalAlpha.invalid
-// Description:
-// Note:
-
-importScripts("/resources/testharness.js");
-importScripts("/common/canvas-tests.js");
-
-var t = async_test("");
-t.step(function() {
-
-var offscreenCanvas = new OffscreenCanvas(100, 50);
-var ctx = offscreenCanvas.getContext('2d');
-
-ctx.globalAlpha = 0.5;
-var a = ctx.globalAlpha; // might not be exactly 0.5, if it is rounded/quantised, so remember for future comparisons
-ctx.globalAlpha = Infinity;
-_assertSame(ctx.globalAlpha, a, "ctx.globalAlpha", "a");
-ctx.globalAlpha = -Infinity;
-_assertSame(ctx.globalAlpha, a, "ctx.globalAlpha", "a");
-ctx.globalAlpha = NaN;
-_assertSame(ctx.globalAlpha, a, "ctx.globalAlpha", "a");
-
-t.done();
-
-});
-done();