Correct the OffscreenCanvas.getContext API

third_party/WebKit/LayoutTests/fast/canvas/OffscreenCanvas-getContext.html

Index: third_party/WebKit/LayoutTests/fast/canvas/OffscreenCanvas-getContext.html
diff --git a/third_party/WebKit/LayoutTests/fast/canvas/OffscreenCanvas-getContext.html b/third_party/WebKit/LayoutTests/fast/canvas/OffscreenCanvas-getContext.html
index 872cb626f43f43e1470237ea25bfc9a8abc7143c..25e16655d90f36e2f2728ee57323e715c5ceb177 100644
--- a/third_party/WebKit/LayoutTests/fast/canvas/OffscreenCanvas-getContext.html
+++ b/third_party/WebKit/LayoutTests/fast/canvas/OffscreenCanvas-getContext.html
@@ -17,6 +17,8 @@ test(function() {
     aCanvas.width = 0; // Zero dimension is allowed.
     assert_equals(aCanvas.width, 0);
 
+    assert_throws(new TypeError(), function() { aCanvas.getContext('bogus'); }); 
+
     // Tests object type of getContext('2d').
     var ctx = aCanvas.getContext('2d');
     assert_true(ctx instanceof OffscreenCanvasRenderingContext2D);