Invoke CommandBar's onItemClick vs item onClick when item has an href set to avoid item is undefined in handler

[KevinTCoughlin]

Pull request checklist

• Addresses an existing issue: Fixes CommandBar Items with href passes undefined item in onClick handler #4556
• Include a change request file using $ npm run change

Description of changes

When a CommandBar's item had an href set we would create an <a> with an onClick bound to that item's onClick handler, rather than invoking the onClick delegate in CommandBar. Because of this onClick was invoked without the backing item for this scenario compared with not having an href set. This change attempts to normalize the click handling by invoking CommandBar's onClick handler which in turn invokes the item's onClick handler.

Focus areas to test

1. Set "Persist Logs" to true in your console or set a break-point at https://github.com/OfficeDev/office-ui-fabric-react/blob/master/packages/office-ui-fabric-react/src/components/CommandBar/CommandBar.tsx#L437
2. If you're checking logs, put a print statement in the onClick callback for the CommandBar example here: https://github.com/OfficeDev/office-ui-fabric-react/blob/master/packages/office-ui-fabric-react/src/components/CommandBar/examples/data-nonFocusable.ts#L25
3. Also set an href on that item, for example { ... href: 'https://www.bing.com/ }
4. Run the solution locally and navigate to http://localhost:4322/#/examples/commandbar. Click on both items with and without href set.
   • If the href IS NOT set the behavior should be as it is on master.
   • If the href IS set, then the item's onClick handler should be invoked with the DOM event and backing item before handling the href. Previously item was undefined.

I feel like this change requires at least an additional test if we wish to merge it so I welcome any guidance there. Will look at what exists in the meantime.

[micahgodbolt]

normalizing click behavior is always a good thing.