Graceful shutdown of UsbService

Graceful shutdown of UsbService.

Enable the exit observer for NOTIFICATION_APP_TERMINATING.

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=221382

[Bei Zhang, 2013-08-29 19:57:17]

Hi Antony,

Please review,

Thanks,
Bei

[asargent_no_longer_on_chrome, 2013-08-30 22:49:30]

lgtm

I assume we already looked at making this a BrowserContextKeyedService but
couldn't because BrowserContextKeyedService's are not singleton's across all
profiles, but rather specific to each profile?

[Bei Zhang, 2013-09-03 09:27:09]

On 2013/08/30 22:49:30, Antony Sargent wrote:
> lgtm
> 
> I assume we already looked at making this a BrowserContextKeyedService but
> couldn't because BrowserContextKeyedService's are not singleton's across all
> profiles, but rather specific to each profile?

That's right. Also, it is hard to destroy a BrowserContextKeyedService on FILE
thread.

[commit-bot: I haz the power, 2013-09-04 22:51:02]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/ikarienator@chromium.org/23483014/10001

[commit-bot: I haz the power, 2013-09-04 23:11:07]

Sorry for I got bad news for ya.
Compile failed with a clobber build on ios_dbg_simulator.
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=ios_dbg_si...
Your code is likely broken or HEAD is junk. Please ensure your
code is not broken then alert the build sheriffs.
Look at the try server FAQ for more details.

[commit-bot: I haz the power, 2013-09-05 09:57:43]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/ikarienator@chromium.org/23483014/10001

[commit-bot: I haz the power, 2013-09-05 11:17:49]

Change committed as 221382