Would you agree that the completely standards compliant, well-understood-by-users back button already replicates that behavior perfectly? Why does the close button have to have a different icon for the exact same purpose, if not to deceive?
Why are you introducing a new and totally unnecessary display element when the back button is built right in to the browser and everyone already knows how it works?
No matter how hard I try, I can't think of any good reason for this banner.
But hey, to the right of that X there sure is a lot of room for placing an adsense ad! What a strange co-incidence.
I can't give you a 100% reason for the X. I know multiple UI versions were tested and this did well.
What an amazing coincidence that this great new tech requires users to remain on google.com "for technical reasons".
We can do it in our native apps, and we do.