Remove "delete this" pattern from UsbContext completely.

Remove "delete this" pattern from UsbContext completely.

This change fixes a much rarer possible use-after-free on USB context
shutdown than commit 3219ab0a9caf806b6dda0fe970869735726a5481. If there
is an event that wakes up the event loop thread after running_ is set to
false then the context could be freed before the attempt to wake it up
with libusb_interrupt_handle_event() is made, instead signaling a freed
object.

This change instead makes it the responsibility of UsbContext to free
the UsbEventHandler once its thread has been joined.

BUG=518749
R=rockot@chromium.org

Committed: https://crrev.com/7ecee444661e8b855c15171615d68162602c39fa
Cr-Commit-Position: refs/heads/master@{#342711}

[Reilly Grant (use Gerrit), 2015-08-10 20:19:01]

Please take a look.

[Ken Rockot(use gerrit already), 2015-08-10 21:51:41]

lgtm

[Reilly Grant (use Gerrit), 2015-08-10 21:52:28]

The CQ bit was checked by reillyg@chromium.org

[commit-bot: I haz the power, 2015-08-10 21:53:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1281373002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1281373002/20001

[commit-bot: I haz the power, 2015-08-10 22:19:00]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-08-10 22:19:33]

Patchset 2 (id:??) landed as
https://crrev.com/7ecee444661e8b855c15171615d68162602c39fa
Cr-Commit-Position: refs/heads/master@{#342711}

[Ken Rockot(use gerrit already), 2015-08-11 22:43:44]

lgtm