quick question for you Eli and Laura, Is it a deal breaker if we can use up/down arrow keys for navigation rather than left/right? 
I'm asking this because it's possible to use off the shelf Polymer paper-listbox to implement the list of avatars. This makes the eng effort MUCH smaller than if I were to implement the behavior from scratch.

up/down seems fine to me if it's significantly easier from eng perspective. Laura - are you okay with that?

I should just add that it's possible to enable left/right on top of up/down as well. meaning right and down move forward. left and up move backward in the list. IMO it was a good experience when I tested it.

That seems reasonable to me. Let's move forward with that. Laura - let us know if you have any concerns.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/41da613e2425d5cd726390dcecdb4102315900c1

commit 41da613e2425d5cd726390dcecdb4102315900c1
Author: mahmadi <mahmadi@chromium.org>
Date: Thu Sep 22 15:46:23 2016

Profile Avatar Selector: Allow arrow keys to be used for moving between avatars.

BUG= 639843 
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2269743002
Cr-Commit-Position: refs/heads/master@{#420355}

[modify] https://crrev.com/41da613e2425d5cd726390dcecdb4102315900c1/chrome/test/data/webui/cr_elements/cr_profile_avatar_selector_tests.js
[modify] https://crrev.com/41da613e2425d5cd726390dcecdb4102315900c1/chrome/test/data/webui/settings/people_page_manage_profile_test.js
[modify] https://crrev.com/41da613e2425d5cd726390dcecdb4102315900c1/ui/webui/resources/cr_elements/cr_profile_avatar_selector/compiled_resources2.gyp
[modify] https://crrev.com/41da613e2425d5cd726390dcecdb4102315900c1/ui/webui/resources/cr_elements/cr_profile_avatar_selector/cr_profile_avatar_selector.html
[add] https://crrev.com/41da613e2425d5cd726390dcecdb4102315900c1/ui/webui/resources/cr_elements/cr_profile_avatar_selector/cr_profile_avatar_selector_grid.html
[add] https://crrev.com/41da613e2425d5cd726390dcecdb4102315900c1/ui/webui/resources/cr_elements/cr_profile_avatar_selector/cr_profile_avatar_selector_grid.js
[modify] https://crrev.com/41da613e2425d5cd726390dcecdb4102315900c1/ui/webui/resources/cr_elements_resources.grdp

fyi, after talking to dbeam@ based on his discussions with dmazzonni@ we decided to have up/down arrow keys to move the focus to the previous/next logical row. This behavior is reflected in the fix.