Make The task manager browser tests for the print preview work under --site-per-process

Make The task manager browser tests for the print preview work under --site-per-process

The newly added test fails under --site-per-process. The reason is we
used to navigate to the NTP which used to create an extra task
(SubframeTask) in the task manager.

Failure link is here:
http://build.chromium.org/p/chromium.fyi/builders/Site%20Isolation%20Linux/builds/3418

TBR=nick@chromium.org, thestig@chromium.org
BUG=470995
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_site_isolation

Committed: https://crrev.com/46d4b6d4fb946f47137458e82390a773e1b5cc32
Cr-Commit-Position: refs/heads/master@{#340805}

[afakhry, 2015-07-28 22:02:14]

The CQ bit was checked by afakhry@chromium.org

[commit-bot: I haz the power, 2015-07-28 22:04:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1256173003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1256173003/1

[afakhry, 2015-07-28 22:25:28]

The CQ bit was unchecked by afakhry@chromium.org

[afakhry, 2015-07-28 22:25:55]

The CQ bit was checked by afakhry@chromium.org

[commit-bot: I haz the power, 2015-07-28 22:27:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1256173003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1256173003/1

[ncarter (slow), 2015-07-28 22:28:18]

lgtm

[commit-bot: I haz the power, 2015-07-28 23:34:50]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-28 23:35:28]

Patchset 1 (id:??) landed as
https://crrev.com/46d4b6d4fb946f47137458e82390a773e1b5cc32
Cr-Commit-Position: refs/heads/master@{#340805}