Make callback arg non-nullable on canvas.toBlob

third_party/WebKit/LayoutTests/fast/canvas/script-tests/canvas-toBlob-invalid.js

+description("Test the handling of invalid arguments in canvas toBlob().");
+
+var canvas = document.createElement('canvas');
+var ctx = canvas.getContext("2d"); 
+ctx.strokeStyle = "red";
+ctx.strokeRect(0, 0, 50, 50);
+
+shouldThrow("canvas.toBlob();");
+shouldThrow("canvas.toBlob(null);");
+shouldThrow("canvas.toBlob(undefined);");
+// Passing the callback argument without blob handle silently fails.
+shouldNotThrow("canvas.toBlob(function() {});");
+
+// Invalid quality argument will fall back to default value
+shouldNotThrow("canvas.toBlob(function(blob) {}, 'image/jpeg', 500)");
+
+