Improve accessibility of user image selection screen.
Gives each image a spoken description. Fixes the ARIA markup so it's
exposed as an interactive listbox when accessibility is turned on.
Also, remaps up/down to be the same as left/right when accessibility
is on, since the 2-dimensional structure is not exposed.
Here's exactly the feedback it gives now, when you tab to the list box
and one item is selected:
"Choose a picture to display for your account on the sign-in screen.
Cupcake, List box 7 of 20".
BUG=
126186,
chromium-os:26999
TEST=manual testing with spoken feedback on
Committed:
https://src.chromium.org/viewvc/chrome?view=rev&revision=150170