Reset webusb detector before browser client

Reset webusb detector before browser client.

This patch modified code to reset webusb detector before browser
client, and added explicit keyword to WebUsbDetector constructor.

BUG=492204
Committed: https://crrev.com/50087e6d65a05df6992a69301af7c273d670c788
Cr-Commit-Position: refs/heads/master@{#348042}

[juncai, 2015-09-09 22:05:34]

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

[juncai, 2015-09-09 22:05:35]

stevenjb@chromium.org and reillyg@chromium.org:
Please review this patch.

[juncai, 2015-09-09 22:06:15]

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

[juncai, 2015-09-09 22:06:15]

thestig@chromium.org: Please review changes in 
chrome/browser/chrome_browser_main.cc

[Lei Zhang, 2015-09-09 22:13:35]

lgtm

[stevenjb, 2015-09-09 22:17:31]

lgtm

[Reilly Grant (use Gerrit), 2015-09-09 23:45:10]

lgtm

[juncai, 2015-09-09 23:55:14]

The CQ bit was checked by juncai@chromium.org

[commit-bot: I haz the power, 2015-09-09 23:55:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1332663002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1332663002/1

[commit-bot: I haz the power, 2015-09-10 00:02:33]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-10 00:03:27]

Patchset 1 (id:??) landed as
https://crrev.com/50087e6d65a05df6992a69301af7c273d670c788
Cr-Commit-Position: refs/heads/master@{#348042}

[commit-bot: I haz the power, 2015-09-23 12:04:58]

Patchset 1 (id:??) landed as
https://crrev.com/50087e6d65a05df6992a69301af7c273d670c788
Cr-Commit-Position: refs/heads/master@{#348042}