MSFT 248 - Add aria-label to close modal dialog button.

Author: gets0ul

__tests__/shared/components/__snapshots__/Modal.jsx.snap

@@ -11,6 +11,7 @@ exports[`Snapshot match 1`] = `
     CONTENT
   </div>
   <button
+    aria-label="Close Dialog"
     className="style__overlay___X12tn"
     onClick={[Function]}
     type="button"

src/shared/components/Modal/index.jsx

@@ -54,6 +54,7 @@ class BaseModal extends React.Component {
             onClick={() => onCancel()}
             className={theme.overlay}
             type="button"
+            aria-label="Close Dialog"
           />
         </React.Fragment>
       ),