[Fixes] #854 Adding a check to see if the element is a prototype of Element before checking computed style

[beekermememe]

Addresses #[854] - Issue on Modal Lock when modal contains web component that contains an iframe

Changes proposed:
Adding a check to see if the element is a prototype of Element before checking computed style

The issue is that as the react-modal is cycling through the elements to determine whether they are tabbable, when it comes to cycling through the modal, it goes from inside to out. As it moves from

<iframe id="wiz"> up to the parent node to

then up the the next parent node

I was able to confirm that the issue is that when it hits the , this is being returned as a ShadowRoot object, which has no parent and is not supported by the window.getComputedStyle (since it can never have style)

Acceptance Checklist:

• [x ] The commit message follows the guidelines in CONTRIBUTING.md.
• [na ] Documentation (README.md) and examples have been updated as needed.
• If this is a code change, a spec testing the functionality has been added.
• [na ] If the commit message has [changed] or [removed], there is an upgrade path above.

[diasbruno]

See comment on patch...

[coveralls]

Coverage decreased (-0.4%) to 84.697% when pulling 42b029d on beekermememe:handle-non-element-in-tabbable-check into 827796d on reactjs:master.

[diasbruno]

@beekermememe Thanks!