Add keyboard navigation to the custom launcher page on app list start page.

Add keyboard navigation to the custom launcher page on app list start page.

This CL makes it possible to navigate to the custom launcher page from
the start page using the keyboard.

Patch written by calamity@chromium.org. Original code review at
https://codereview.chromium.org/941213003/.

BUG=458884
R=mgiuca@chromium.org

Committed: https://crrev.com/7b42856f352f4a8eb07682d082fb8d553bba81b8
Cr-Commit-Position: refs/heads/master@{#317305}

[Matt Giuca, 2015-02-20 12:06:48]

mgiuca@chromium.org changed reviewers:
+ calamity@chromium.org

[Matt Giuca, 2015-02-20 12:06:49]

Alternatively, fix the conflict in https://codereview.chromium.org/941213003/
and commit that.

[tapted, 2015-02-20 12:16:26]

tapted@chromium.org changed reviewers:
+ tapted@chromium.org

[tapted, 2015-02-20 12:16:26]

lgtm

[Matt Giuca, 2015-02-20 12:17:35]

Oh hi Trent. Thanks!

[Matt Giuca, 2015-02-20 12:18:22]

The CQ bit was checked by mgiuca@chromium.org

[commit-bot: I haz the power, 2015-02-20 12:18:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/924193005/1

[commit-bot: I haz the power, 2015-02-20 13:07:47]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-20 13:08:39]

Patchset 1 (id:??) landed as
https://crrev.com/7b42856f352f4a8eb07682d082fb8d553bba81b8
Cr-Commit-Position: refs/heads/master@{#317305}