[Mac] Prevent using tab context menu after tab closed.
[Mac] Prevent using tab context menu after tab closed.

JavaScript can close windows while in the event loop processing a
right-click menu.  This change prevents sending messages to the tab
controller after it has been closed.

BUG=25462, 25465
TEST=See bug for test.html.
TEST=Run test.html, right-click tab, after close all items should be grayed out.
TEST=Run test.html, click tab's close button and hold until tab closes.  Should not crash on release.

Review URL: http://codereview.chromium.org/335048

git-svn-id: http://src.chromium.org/svn/trunk/src@30269 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 61038d7c18b81a395b1f3b697cb6d80616ef6aef
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser