Handle exception when user call canvas.transferControlToOffscreen() twice

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

Index: third_party/WebKit/LayoutTests/fast/canvas/OffscreenCanvas-transferControlToOffscreen.html
diff --git a/third_party/WebKit/LayoutTests/fast/canvas/OffscreenCanvas-transferControlToOffscreen.html b/third_party/WebKit/LayoutTests/fast/canvas/OffscreenCanvas-transferControlToOffscreen.html
index e541808549b9485387e0a383c4e305a6bc0debf4..e961996b03fe8d9a6110723384fe7653b2c85500 100644
--- a/third_party/WebKit/LayoutTests/fast/canvas/OffscreenCanvas-transferControlToOffscreen.html
+++ b/third_party/WebKit/LayoutTests/fast/canvas/OffscreenCanvas-transferControlToOffscreen.html
@@ -28,13 +28,19 @@ test(function() {
 }, "Tests whether transferControlToOffscreen can be run correctly.");
 
 test(function() {
-    var canvas2 = createCanvas(50, 50);
-    var offscreenCanvas2;
-    var ctx = canvas2.getContext("2d");
+    var canvas2a = createCanvas(50, 50);
+    var offscreenCanvas2a;
+    var ctx = canvas2a.getContext("2d");
     assert_throws("InvalidStateError", function() {
-        offscreenCanvas2 = canvas2.transferControlToOffscreen();
+        offscreenCanvas2a = canvas2a.transferControlToOffscreen();
         assert_false("transferControlToOffscreen from a canvas with context didn't throw an exception.");
     }, "transferControlToOffscreen from a canvas with context throws an exception");
+
+    var canvas2b = createCanvas(10, 10);
+    var offscreenCanvas2b = canvas2b.transferControlToOffscreen();
+    assert_throws("InvalidStateError", function() {
+        var anotherOffscreenCanvas = canvas2b.transferControlToOffscreen();
+    }, "canvas.transferControlToOffscreen() is not allowed to called more than once for the same canvas.");
 }, "Tests whether transferControlToOffscreen throws exception correctly."); 
 
 test(function() {