Fix handle_watcher when start is called twice on the same handle.

Fix handle_watcher when start is called twice on the same handle.

This carries over a fix by qsr@ for the mojo helper handle_watcher
landed in the mojo repo in https://codereview.chromium.org/781753004.

The fix allows the network service apptests not to crash when built
against the Chromium copy of mojo/common.

BUG=450356
Committed: https://crrev.com/5e9a3adac10fdf8d172de5aa0d7ec9a7bbb1e441
Cr-Commit-Position: refs/heads/master@{#313261}

[ppi, 2015-01-26 15:33:59]

ppi@chromium.org changed reviewers:
+ jamesr@chromium.org, qsr@chromium.org

[ppi, 2015-01-26 15:36:41]

Hi Ben, James,

ptal.

[qsr, 2015-01-26 15:40:18]

lgtm

[jamesr, 2015-01-26 20:37:18]

lgtm, although this is a little bit bogus in that it's perfectly valid to have
simultaneous waits on the same handle for different signal sets although this
class doesn't support that.

[ppi, 2015-01-27 12:21:12]

The CQ bit was checked by ppi@chromium.org

[commit-bot: I haz the power, 2015-01-27 12:22:31]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/878513003/1

[commit-bot: I haz the power, 2015-01-27 12:25:29]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-27 12:26:46]

Patchset 1 (id:??) landed as
https://crrev.com/5e9a3adac10fdf8d172de5aa0d7ec9a7bbb1e441
Cr-Commit-Position: refs/heads/master@{#313261}