Update commit reference for Selenium atoms (follow-up from issue #1773)

[whimboo]

To fix a regression in Firefox we had to merge the fix for issue #1773 in time. As such we had to delay the landing/fix for SeleniumHQ/selenium#13206. Now that this fix (Fix "Get Text" to retrieve the correct value for ShadowRoot using slot.) has also been landed we should bump the Selenium atoms again to the revision 21560a497470f0c769931c17fa9803c6cddf6f1c. No other changes have landed since then in dom.js.