Prevent crash when calling text-related methods on a 2D canvas in a frame-less document.

LayoutTests/fast/canvas/script-tests/canvas-frameless-document-text.js

+description("This verifies that the browser does not crash when drawing text to a canvas in a frame-less document.");
+
+// It is not clear frome the spec whether this is supposed to work and how.
+// Therefore, we do not validate the rendering results. We just make sure
+// this does not crash the browser.
+var canvas1 = document.createElement('canvas');
+var ctx1 = canvas1.getContext('2d');
+var htmlDoc = document.implementation.createHTMLDocument('', '', null);
+htmlDoc.adoptNode(canvas1);
+var canvas2 = htmlDoc.createElement('canvas');
+var ctx2 = canvas2.getContext('2d');
+
+ctx1.font = 'italic 30px Arial';
+ctx2.font = 'italic 30px Arial';
+ctx1.fillText('Text1', 0, 30);
+ctx2.fillText('Text1', 0, 30);
+ctx1.strokeText('Text2', 0, 60);
+ctx2.strokeText('Text2', 0, 60);
+ctx1.measureText('Text3');
+ctx2.measureText('Text3');