3805 [google_sign_in] Fix “pick account” on iOS (cla: yes, waiting for tree to go green, p: google_sign_in, platform-ios)

3816 [google_sign_in_web] fix README typos. (cla: yes, p: google_sign_in, platform-web)

3819 Add a todo WRT correctly setting the X-Goog-AuthUser header (cla: yes, p: google_sign_in)