Make DocumentModeTest less flaky

Make DocumentModeTest less flaky

Android likes to hold onto Activities while Chrome is in the foreground.
Send the user home before closing everything.

BUG=451453
Committed: https://crrev.com/984f779607cc0ca56de0326dcf0f9ee86c8486ef
Cr-Commit-Position: refs/heads/master@{#333792}

[gone, 2015-06-10 18:45:46]

dfalcantara@chromium.org changed reviewers:
+ mariakhomenko@chromium.org

[gone, 2015-06-10 18:45:46]

Saw the Incognito launching test acting up locally.  Saw that the
MultiActivityTestBase was failing to close all the Activities.

[Maria, 2015-06-10 20:09:43]

lgtm

[gone, 2015-06-10 20:10:35]

The CQ bit was checked by dfalcantara@chromium.org

[commit-bot: I haz the power, 2015-06-10 20:11:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1176043002/1

[commit-bot: I haz the power, 2015-06-10 20:37:37]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-10 20:38:39]

Patchset 1 (id:??) landed as
https://crrev.com/984f779607cc0ca56de0326dcf0f9ee86c8486ef
Cr-Commit-Position: refs/heads/master@{#333792}