Fix crash on app list start page contents not existing.

Fix crash on app list start page contents not existing.

This CL adds a null check around an access to the app list web contents.

BUG=460388
Committed: https://crrev.com/386f2bcf06d5fb74c176de864190a1c555ad618f
Cr-Commit-Position: refs/heads/master@{#317546}

[calamity, 2015-02-23 04:38:48]

Patchset #1 (id:1) has been deleted

[calamity, 2015-02-23 04:39:31]

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

[Matt Giuca, 2015-02-23 04:40:36]

lgtm

[calamity, 2015-02-23 05:27:32]

The CQ bit was checked by calamity@chromium.org

[commit-bot: I haz the power, 2015-02-23 05:29:33]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/949823002/20001

[commit-bot: I haz the power, 2015-02-23 05:43:23]

Committed patchset #1 (id:20001)

[commit-bot: I haz the power, 2015-02-23 05:44:00]

Patchset 1 (id:??) landed as
https://crrev.com/386f2bcf06d5fb74c176de864190a1c555ad618f
Cr-Commit-Position: refs/heads/master@{#317546}