It's a bit of a hassle but you have to go to View In (top-right corner) -> Git Revision Log, copy the SHA1 of the topmost commit, go back, View In -> Git, and replace "master" in the URL with the copied SHA1.
(It appears Gitiles doesn't support using the SHA1 of the tip of the tree as showed in the latter page, which is why you have to get the SHA1 of the latest commit to that file from the former page instead.)
It's a bit of a hassle but you have to go to View In (top-right corner) -> Git Revision Log, copy the SHA1 of the topmost commit, go back, View In -> Git, and replace "master" in the URL with the copied SHA1.
(It appears Gitiles doesn't support using the SHA1 of the tip of the tree as showed in the latter page, which is why you have to get the SHA1 of the latest commit to that file from the former page instead.)