From review of https://codereview.chromium.org/2545723003, it would nice to be able to update the user image only when necessary. That is, only send the image over mojo on initial update and when user image is actually changed.

We should not send the image data when it is not change, e.g. when the active user is changed.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/66e4177886b272950cafc7dabed256025dcf7013

commit 66e4177886b272950cafc7dabed256025dcf7013
Author: xiyuan <xiyuan@chromium.org>
Date: Tue Dec 13 21:57:58 2016

chromeos: Replace user image notifications with observer

BUG= 670422 

Review-Url: https://codereview.chromium.org/2568973002
Cr-Commit-Position: refs/heads/master@{#438302}

[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/chrome/browser/chrome_notification_types.h
[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/chrome/browser/chromeos/login/screens/user_image_screen.cc
[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/chrome/browser/chromeos/login/screens/user_image_screen.h
[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/chrome/browser/chromeos/login/signin_screen_controller.cc
[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/chrome/browser/chromeos/login/signin_screen_controller.h
[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/chrome/browser/chromeos/login/users/avatar/user_image_manager_browsertest.cc
[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/chrome/browser/chromeos/login/users/avatar/user_image_manager_impl.cc
[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/chrome/browser/chromeos/login/users/avatar/user_image_manager_impl.h
[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/chrome/browser/chromeos/login/users/avatar/user_image_sync_observer.cc
[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/chrome/browser/chromeos/login/users/avatar/user_image_sync_observer.h
[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/chrome/browser/chromeos/login/users/fake_chrome_user_manager.cc
[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/chrome/browser/chromeos/login/users/fake_chrome_user_manager.h
[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/chrome/browser/ui/ash/system_tray_delegate_chromeos.cc
[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/chrome/browser/ui/ash/system_tray_delegate_chromeos.h
[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/chrome/browser/ui/webui/options/browser_options_handler.cc
[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/chrome/browser/ui/webui/options/browser_options_handler.h
[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/chrome/browser/ui/webui/options/chromeos/change_picture_options_handler.cc
[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/chrome/browser/ui/webui/options/chromeos/change_picture_options_handler.h
[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/chrome/browser/ui/webui/settings/chromeos/change_picture_handler.cc
[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/chrome/browser/ui/webui/settings/chromeos/change_picture_handler.h
[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/chrome/browser/ui/webui/settings/profile_info_handler.cc
[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/chrome/browser/ui/webui/settings/profile_info_handler.h
[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/components/user_manager/user_manager.cc
[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/components/user_manager/user_manager.h
[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/components/user_manager/user_manager_base.cc
[modify] https://crrev.com/66e4177886b272950cafc7dabed256025dcf7013/components/user_manager/user_manager_base.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/64fa7e77502cc896a5ba7d8eab1dd19447fda467

commit 64fa7e77502cc896a5ba7d8eab1dd19447fda467
Author: xiyuan <xiyuan@chromium.org>
Date: Fri Jan 13 01:23:29 2017

ash: SessionControllerClient observes user image change

BUG= 670422 

Review-Url: https://codereview.chromium.org/2619653002
Cr-Commit-Position: refs/heads/master@{#443431}

[modify] https://crrev.com/64fa7e77502cc896a5ba7d8eab1dd19447fda467/chrome/browser/chromeos/file_manager/file_manager_browsertest.cc
[modify] https://crrev.com/64fa7e77502cc896a5ba7d8eab1dd19447fda467/chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager_browsertest.cc
[modify] https://crrev.com/64fa7e77502cc896a5ba7d8eab1dd19447fda467/chrome/browser/download/notification/download_notification_browsertest.cc
[modify] https://crrev.com/64fa7e77502cc896a5ba7d8eab1dd19447fda467/chrome/browser/ui/ash/chrome_new_window_client_browsertest.cc
[modify] https://crrev.com/64fa7e77502cc896a5ba7d8eab1dd19447fda467/chrome/browser/ui/ash/session_controller_client.cc
[modify] https://crrev.com/64fa7e77502cc896a5ba7d8eab1dd19447fda467/chrome/browser/ui/ash/session_controller_client.h

Next step would be tracked with  issue 648964  when ash changes to use data from SessionController for UI.