Implement preview tutorials on Mac ...x20web.corp.google.com/users/gu/guohui/www/preview_mac.png ...x20web.corp.google.com/users/gu/guohui/www/preview_enabled_mac.png
fix various mac bugs in the new avatar bubble ...x20web.corp.google.com/users/gu/guohui/www/account_removal_card_revised.png ...x20web.corp.google.com/users/gu/guohui/www/tutorial_button_styled.png
Update avatar tutorial per newspec https://x20web.corp.google.com/~guohui/tutorial_caret.png and ...x20web.corp.google.com/users/gu/guohui/www/tutorial_caret_mac.png
implementaccount removal card and add title to signin... ...https://x20web.corp.google.com/~guohui/remove_secondary_mac.png https://x20web.corp.google.com/~guohui/remove_primary_mac.png https://x20web.corp.google.com/~guohui/signin_title_mac.png