Avoid a race between libusb_submit_transfer and libusb_free_transfer.

Avoid a race between libusb_submit_transfer and libusb_free_transfer.

The tail end of libusb_submit_transfer assumes that the transfer object
is still valid. It may in fact have already been freed by the transfer
completion callback if that call occurs on a different thread. As a
simply workaround (since the libusb code will be going away soon) this
patch simply posts the transfer deletion back to the thread where
libusb_submit_transfer is called so that they are serialized.

BUG=487712
Committed: https://crrev.com/ce48c8a39c88803c6ca73c1fb4cf07f73fec7939
Cr-Commit-Position: refs/heads/master@{#329953}

[Reilly Grant (use Gerrit), 2015-05-14 19:41:51]

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

[Reilly Grant (use Gerrit), 2015-05-14 19:41:52]

Please take a look.

[Reilly Grant (use Gerrit), 2015-05-14 21:04:05]

The CQ bit was checked by reillyg@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-05-14 21:06:27]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1126203008/1

[Ken Rockot(use gerrit already), 2015-05-14 21:41:18]

lgtm

[Reilly Grant (use Gerrit), 2015-05-14 21:43:35]

The CQ bit was unchecked by reillyg@chromium.org

[Reilly Grant (use Gerrit), 2015-05-14 21:43:41]

The CQ bit was checked by reillyg@chromium.org

[commit-bot: I haz the power, 2015-05-14 21:44:16]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1126203008/1

[commit-bot: I haz the power, 2015-05-14 22:27:21]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-14 22:28:21]

Patchset 1 (id:??) landed as
https://crrev.com/ce48c8a39c88803c6ca73c1fb4cf07f73fec7939
Cr-Commit-Position: refs/heads/master@{#329953}