Convert OffscreenCanvas-getContext tests to testharness.

third_party/WebKit/LayoutTests/fast/canvas/OffscreenCanvas-getContext-in-worker.js

+importScripts("../../resources/testharness.js");

[fs, 2016/08/25 08:14:58]

Could possibly put this file in a resources/ or script-tests/ subdirectory, but
I'll leave that for others to decide (this should work as-is AFAIK.)

+test(function() {
+    var aCanvas = new OffscreenCanvas(50, 50);
+    var bCanvas = new OffscreenCanvas(50, 50);
+    var ctx1 = aCanvas.getContext('2d');
+    var ctx2 = bCanvas.getContext('webgl');
+    assert_true(ctx1 instanceof OffscreenCanvasRenderingContext2D);
+    assert_true(ctx2 instanceof WebGLRenderingContext);
+});
+done();