Fix leak of PlatformHandleDispatchers in Mojo IPC

Fix leak of PlatformHandleDispatchers in Mojo IPC

PassWrappedPlatformHandle does not close the MojoHandle.
The destructor of ScopedHandleBase will however close it.
The problem with this code was that it called release()
on the ScopedHandle, preventing the destructor from being
called, and thus also the MojoHandle from being closed.

This leaves the PlatformHandleDispatcher associated with
the MojoHandle alive in mojo::system::Core::handle_table_

By calling reset() instead the MojoHandle is closed, and
the leak is fixed.

R=morrita@chromium.org, viettrungluu@chromium.org, agl@chromium.org

BUG=None

Committed: https://crrev.com/b2134362e27459f1098a35effedad91e2d553192
Cr-Commit-Position: refs/heads/master@{#334197}

[rogerj, 2015-06-12 09:50:18]

The CQ bit was checked by rogerj@opera.com to run a CQ dry run

[commit-bot: I haz the power, 2015-06-12 09:50:50]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1174423002/1

[commit-bot: I haz the power, 2015-06-12 09:50:52]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-06-12 09:50:53]

Dry run: No LGTM from a valid reviewer yet. Only full committers are accepted.
Even if an LGTM may have been provided, it was from a non-committer,
_not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[Hajime Morrita, 2015-06-12 17:14:46]

LGTM. Thanks much for finding and fixing this, Roger!
I wasn't aware that PassWrappedPlatformHandle() just pulles the platform handle
out but
doesn't close the Mojo handle :-(

[Hajime Morrita, 2015-06-12 17:14:50]

The CQ bit was checked by morrita@chromium.org

[commit-bot: I haz the power, 2015-06-12 17:16:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1174423002/1

[commit-bot: I haz the power, 2015-06-12 18:17:50]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-12 18:18:50]

Patchset 1 (id:??) landed as
https://crrev.com/b2134362e27459f1098a35effedad91e2d553192
Cr-Commit-Position: refs/heads/master@{#334197}