[Feedback] Add support for using callbacks for ConnectivityTask

[Feedback] Add support for using callbacks for ConnectivityTask

Currently the ConnectivityTask for gathering data about connectivity
only supported polling for the data.

This CL changes it to also support an optional callback which will be
called when all results have been gathered, or the timeout has been
hit.

BUG=386395
Committed: https://crrev.com/6181f2772f5d0c0696e9cdff1b73d3b69eb224f4
Cr-Commit-Position: refs/heads/master@{#337109}

[nyquist, 2015-06-30 21:02:45]

nyquist@chromium.org changed reviewers:
+ dfalcantara@chromium.org

[nyquist, 2015-06-30 21:02:46]

dfalcantara: PTAL

[gone, 2015-07-01 15:58:42]

lgtm

[nyquist, 2015-07-01 19:53:21]

The CQ bit was checked by nyquist@chromium.org

[nyquist, 2015-07-01 19:53:22]

The patchset sent to the CQ was uploaded after l-g-t-m from
dfalcantara@chromium.org
Link to the patchset: https://codereview.chromium.org/1215233002/#ps30004
(title: "Rebased")

[commit-bot: I haz the power, 2015-07-01 19:53:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1215233002/30004

[commit-bot: I haz the power, 2015-07-01 20:33:28]

Committed patchset #3 (id:30004)

[commit-bot: I haz the power, 2015-07-01 20:34:26]

Patchset 3 (id:??) landed as
https://crrev.com/6181f2772f5d0c0696e9cdff1b73d3b69eb224f4
Cr-Commit-Position: refs/heads/master@{#337109}