Check that extensions::HidDeviceManager is used from the FILE thread.

Check that extensions::HidDeviceManager is used from the FILE thread.

Commit 7fb8b9950 attempted to enable an unused thread check in
extensions::HidDeviceManager but this was not correct because while this
class is normally used from the FILE thread this is not where it is
created (issue 417939). This change adopts the pattern used in
extensions::ApiResourceManager where the BrowserContextKeyedAPI
implements its own thread assertions.

BUG=

Committed: https://crrev.com/ea245886e3af507367cae1d9d766755f4384d588
Cr-Commit-Position: refs/heads/master@{#297013}

[Reilly Grant (use Gerrit), 2014-09-26 17:10:38]

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

[Ken Rockot(use gerrit already), 2014-09-26 18:48:55]

lgtm

[Reilly Grant (use Gerrit), 2014-09-26 19:51:38]

The CQ bit was checked by reillyg@chromium.org

[commit-bot: I haz the power, 2014-09-26 19:52:30]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/604843003/1

[commit-bot: I haz the power, 2014-09-26 19:56:39]

Committed patchset #1 (id:1) as 9abe5191b522cba4823039cb629e1a6a59815d98

[commit-bot: I haz the power, 2014-09-26 19:57:16]

Patchset 1 (id:??) landed as
https://crrev.com/ea245886e3af507367cae1d9d766755f4384d588
Cr-Commit-Position: refs/heads/master@{#297013}