[tabbed options] more work on content settings exceptions lists

- tie "edit" mode to "selected"
-- selecting enters into edit mode
-- blurring exits edit mode and deselects
- fix up layout within rows (faux columns)
- add a special "add new exception" row (this should be at the bottom of the table, but it's currently at the top)
- change behavior when the user attempts to commit an invalid exception. Instead of trying to regrab focus, just revert the row to its previous state.

TODO: list should be sized dynamically, i.e. no scrollbar.
TODO: list should be in sub-sub dialog
TODO: rows should have delete X

BUG=64153
TEST=manual

Review URL: http://codereview.chromium.org/5699004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@69332 0039d316-1c4b-4281-b951-d872f2087c98