Fix experimental app list's start page logo not showing in Guest Mode.

Fix experimental app list's start page logo not showing in Guest Mode.

This CL fixes an issue with the app list not showing the start page
logo in ChromeOS Guest Mode. This was happening because no start page
service was being created for incognito contexts.

This will also cause voice search to be enabled in the app list in
Guest Mode.

BUG=440484
Committed: https://crrev.com/64121b7f8233a309b0f818269a5049976ba37e3c
Cr-Commit-Position: refs/heads/master@{#315272}

[calamity, 2015-02-09 02:30:06]

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

[Matt Giuca, 2015-02-09 03:05:06]

lgtm

[calamity, 2015-02-09 03:54:44]

The CQ bit was checked by calamity@chromium.org

[commit-bot: I haz the power, 2015-02-09 03:55:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/879033003/1

[commit-bot: I haz the power, 2015-02-09 04:53:31]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-02-09 04:53:32]

Try jobs failed on following builders:
  linux_chromium_chromeos_rel_ng on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[calamity, 2015-02-09 05:49:53]

New patchsets have been uploaded after l-g-t-m from mgiuca@chromium.org

[calamity, 2015-02-09 06:12:36]

The CQ bit was checked by calamity@chromium.org

[commit-bot: I haz the power, 2015-02-09 06:12:40]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/879033003/20001

[commit-bot: I haz the power, 2015-02-09 06:33:02]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-02-09 06:33:38]

Patchset 2 (id:??) landed as
https://crrev.com/64121b7f8233a309b0f818269a5049976ba37e3c
Cr-Commit-Position: refs/heads/master@{#315272}