Clear the cancel callback before calling it.

Clear the cancel callback before calling it.

As the completion callback is called externally it should be cleared first, to
avoid possible double cancellation.

TEST=None
BUG=None

Committed: https://crrev.com/41d8d3c2026d46f860694dc0fc51e61543be8adf
Cr-Commit-Position: refs/heads/master@{#311422}

[mtomasz, 2015-01-14 06:33:10]

mtomasz@chromium.org changed reviewers:
+ hirono@chromium.org

[mtomasz, 2015-01-14 06:33:17]

@hirono: PTAL. Thanks.

[hirono, 2015-01-14 06:39:51]

lgtm!

[mtomasz, 2015-01-14 06:52:26]

The CQ bit was checked by mtomasz@chromium.org

[commit-bot: I haz the power, 2015-01-14 06:53:09]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/838023003/1

[commit-bot: I haz the power, 2015-01-14 06:55:07]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-14 06:55:58]

Patchset 1 (id:??) landed as
https://crrev.com/41d8d3c2026d46f860694dc0fc51e61543be8adf
Cr-Commit-Position: refs/heads/master@{#311422}