Add some layout tests about behavior on undefined and null.

LayoutTests/fast/canvas/script-tests/canvas-path-context-stroke.js

 description("Series of tests to ensure stroke() works with optional path parameter.");
 
 var ctx = document.getElementById('canvas').getContext('2d');
 
 function pixelDataAtPoint() {
   return ctx.getImageData(75, 75, 1, 1).data;
 }
 
 function checkResult(expectedColors, sigma) {
     for (var i = 0; i < 4; i++)
@@ skipping 29 matching lines @@
 // Execute test.
 function prepareTestScenario() {
     var path = new Path2D();
     drawRectangleOn(path);
 
     testStrokeWith();
     testStrokeWith(path);
 
     // Test exception cases.
     shouldThrow("ctx.stroke(null)");
+    shouldThrow("ctx.stroke(undefined)");
     shouldThrow("ctx.stroke([])");
     shouldThrow("ctx.stroke({})");
 }
 
 // Run test and allow variation of results.
 prepareTestScenario();