Clean up message_box_handler.h.
Clean up message_box_handler.h.

BUG=none.
TEST=All build bots should still be happy.

Review URL: http://codereview.chromium.org/3619001

git-svn-id: http://src.chromium.org/svn/trunk/src@61393 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 85895168a026c84524d7aa5c2c3e4332ca5ff8f8
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