[Mac] Fix Cmd-W closing whole window under fullscreen.

Fix Cmd-W closing whole window under fullscreen.

This got broken by http://crrev.com/116144, which made Cmd-W
(close tab) route to -performClose:.

However, FullscreenWindow had an override for -performClose:,
resulting in the logic to route Cmd-W to -commandDispatch: not
taking place for fullscreen windows.

Also fixes -[BWC validateUserInterfaceItem:] to correctly disable
IDC_CLOSE_TAB menu items, which was also broken in the same
revision.

BUG=109793
TEST=1. Open a fullscreen window with multiple tabs. Hit Cmd-W. A single tab should close.
2. Open a window that doesn't have tabs - e.g. Window -> Task Manager. File -> Close Tab
should be disabled.

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=117681

[Alexei Svitkine (slow), 2012-01-13 18:16:29]

ping

[Robert Sesek, 2012-01-13 18:46:38]

lgtm.

Sorry for the delay.

[commit-bot: I haz the power, 2012-01-13 18:47:18]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/asvitkine@chromium.org/9187063/5001

[commit-bot: I haz the power, 2012-01-13 20:30:49]

Change committed as 117681