Revert exposing <dialog> in WebUI

Revert exposing <dialog> in WebUI

The plan to use <dialog> in WebUI didn't work out. It was risky to use
an experimental feature in Settings, especially as Chrome for iOS won't
be able to use it.

BUG=276825
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=250986

[jochen (gone - plz use gerrit), 2014-02-12 08:53:46]

lgtm

[falken, 2014-02-12 08:56:38]

The CQ bit was checked by falken@chromium.org

[commit-bot: I haz the power, 2014-02-12 08:59:00]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/falken@chromium.org/156853006/1

[commit-bot: I haz the power, 2014-02-12 11:36:58]

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

[commit-bot: I haz the power, 2014-02-12 11:36:59]

Retried try job too often on win_rel for step(s) browser_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=win_rel&nu...

[falken, 2014-02-13 02:05:02]

The CQ bit was checked by falken@chromium.org

[commit-bot: I haz the power, 2014-02-13 02:16:42]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/falken@chromium.org/156853006/1

[commit-bot: I haz the power, 2014-02-13 14:30:57]

Change committed as 250986