Fixed crash with Multisample observer

Fixed crash with Multisample observer

BUG=333040
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=170253

[bajones, 2014-03-27 23:36:33]

This took a stupidly long time to find this, but fortunately the solution was
pretty simple. If the multisample changed signal was received after the page was
disconnected from the document and the WebGL context was destroyed we would
crash. This makes sure that the change listeners are removed on page detachment.

[Ken Russell (switch to Gerrit), 2014-03-27 23:45:58]

The code change LGTM, but can a test be added for this?

[bajones, 2014-03-28 00:32:46]

On 2014/03/27 23:45:58, Ken Russell wrote:
> The code change LGTM, but can a test be added for this?

I totally agree, but given the crashy nature of this issue I'm going to land the
fix immediately.

[bajones, 2014-03-28 00:32:51]

The CQ bit was checked by bajones@chromium.org

[commit-bot: I haz the power, 2014-03-28 00:33:47]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/bajones@chromium.org/214413010/1

[commit-bot: I haz the power, 2014-03-28 01:36:57]

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

[commit-bot: I haz the power, 2014-03-28 01:36:57]

Try jobs failed on following builders:
  tryserver.blink on linux_blink_dbg

[bajones, 2014-03-28 01:47:44]

The CQ bit was checked by bajones@chromium.org

[commit-bot: I haz the power, 2014-03-28 01:47:50]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/bajones@chromium.org/214413010/1

[commit-bot: I haz the power, 2014-03-28 02:51:26]

Change committed as 170253