Fix WebUSB notification on Chrome OS with no windows open.

Fix WebUSB notification on Chrome OS with no windows open.

This patch fixes WebUSB notifications so that Chrome does not crash when
opening the landing page URL on Chrome OS when there are no windows
open. This is done with a ScopedTabbedBrowserDisplayer which will create
a new browser window if one is not visible. The notification strings are
also updated to match the most recent mocks.

BUG=492204
Committed: https://crrev.com/48291d82d4080d05975da8949b5cd7e92d7a0a1e
Cr-Commit-Position: refs/heads/master@{#350360}

[Reilly Grant (use Gerrit), 2015-09-22 21:37:39]

reillyg@chromium.org changed reviewers:
+ juncai@chromium.org

[Reilly Grant (use Gerrit), 2015-09-22 21:37:56]

Please take a look.

[juncai, 2015-09-22 22:00:12]

lgtm

[Reilly Grant (use Gerrit), 2015-09-22 22:16:40]

The CQ bit was checked by reillyg@chromium.org

[commit-bot: I haz the power, 2015-09-22 22:17:31]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1360873003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1360873003/1

[commit-bot: I haz the power, 2015-09-22 22:39:13]

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

[commit-bot: I haz the power, 2015-09-22 22:39:15]

Try jobs failed on following builders:
  mac_chromium_rel_ng on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_rel_...)

[Reilly Grant (use Gerrit), 2015-09-22 23:53:49]

The CQ bit was checked by reillyg@chromium.org

[Reilly Grant (use Gerrit), 2015-09-22 23:53:49]

The patchset sent to the CQ was uploaded after l-g-t-m from juncai@chromium.org
Link to the patchset: https://codereview.chromium.org/1360873003/#ps20001
(title: "Update expected strings.")

[commit-bot: I haz the power, 2015-09-22 23:55:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1360873003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1360873003/20001

[commit-bot: I haz the power, 2015-09-23 05:22:33]

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

[commit-bot: I haz the power, 2015-09-23 05:22:33]

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

[Reilly Grant (use Gerrit), 2015-09-23 18:29:45]

The CQ bit was checked by reillyg@chromium.org

[commit-bot: I haz the power, 2015-09-23 18:30:46]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1360873003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1360873003/20001

[commit-bot: I haz the power, 2015-09-23 20:28:30]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-09-23 20:30:19]

Patchset 2 (id:??) landed as
https://crrev.com/48291d82d4080d05975da8949b5cd7e92d7a0a1e
Cr-Commit-Position: refs/heads/master@{#350360}