Clear the onMessage and onDisconnect listeners when their Extension Port is destroyed.

Clear the onMessage and onDisconnect listeners when their Extension Port is destroyed.

I was ambitious in r325585 to remove this behavior, asserting that the Port's
owner should clear its own listeners. This is an optimistic expectation, and a
backwards incompatible one at any rate.

BUG=486715
R=rockot@chromium.org

Committed: https://crrev.com/1be1629824f63c62466b44fc90d40101fa9d495a
Cr-Commit-Position: refs/heads/master@{#329265}

[Ken Rockot(use gerrit already), 2015-05-11 20:57:09]

lgtm

[not at google - send to devlin, 2015-05-11 21:16:03]

The CQ bit was checked by kalman@chromium.org

[commit-bot: I haz the power, 2015-05-11 21:16:40]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1135123002/1

[commit-bot: I haz the power, 2015-05-11 23:04:02]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-11 23:04:44]

Patchset 1 (id:??) landed as
https://crrev.com/1be1629824f63c62466b44fc90d40101fa9d495a
Cr-Commit-Position: refs/heads/master@{#329265}