Mac: fix Text Encoding menu/submenu

(1) Clicking on the Text Encoding menu item should display the submenu, not try to execute a nonexistent command.
(2) The Text Encoding menu item shouldn't be disabled, but instead the submenu's contents (so that the submenu is always visible). See Apple's HIG:
<http://developer.apple.com/mac/library/DOCUMENTATION/UserExperience/Conceptual/AppleHIGuidelines/XHIGMenus/XHIGMenus.html#//apple_ref/doc/uid/TP30000356-TPXREF122>.

Changes to MainMenu.xib: disconnected |-commandDispatch:| from the Text Encoding menu item.

BUG=25957,26593
TEST=Check that the Text Encoding submenu is always available, with its contents disabled on the NTP (for example) and enabled (and working) on a normal web page. Check that clicking (and releasing) on the Text Encoding menu item itself shows the submenu.

Review URL: http://codereview.chromium.org/345050

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@30836 0039d316-1c4b-4281-b951-d872f2087c98