[Mac] The Switch Person button can be moar accessible

[Mac] The Switch Person button can be moar accessible

It now reads "<name> switch person button" instead of "<name> button" if
you have multiple profiles, and "switch person button" instead of absolute
silence if you only have a single, local profile.

BUG=450419
Committed: https://crrev.com/8df8052a7ee753b9cab9bccb4f5d0a53d6534f46
Cr-Commit-Position: refs/heads/master@{#313848}

[noms (inactive), 2015-01-29 23:00:58]

noms@chromium.org changed reviewers:
+ asvitkine@chromium.org

[noms (inactive), 2015-01-29 23:00:59]

Making accessibility a thing, one button at a time. Please take a look! :)

[Alexei Svitkine (slow), 2015-01-30 00:45:52]

lgtm

[noms (inactive), 2015-01-30 01:04:40]

The CQ bit was checked by noms@chromium.org

[commit-bot: I haz the power, 2015-01-30 01:06:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/890693003/1

[commit-bot: I haz the power, 2015-01-30 01:10:18]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-30 01:11:03]

Patchset 1 (id:??) landed as
https://crrev.com/8df8052a7ee753b9cab9bccb4f5d0a53d6534f46
Cr-Commit-Position: refs/heads/master@{#313848}