What steps will reproduce the problem?
(1) On OOBE "Chrome OS terms" page, click "System security setting" link
(2) In "System security setting" dialog, Press TAB repeatedly

What is the expected result?
Nothing happens (focus shouldn't leave the dialog)

What happens instead?
Focus moves to SystemTray > Terms textbox > "System security setting" link > etc

When elements of the Terms page are focused, they may be activated via keyboard.  For example, you may TAB focus onto the "Accept and continue" button, press ENTER, and move on to GAIA login, all with the "System security setting" dialog still on top, and everything behind it dimmed.  When this dialog is present, focus should not leave the OK button.

There used to be code in place to make this dialog modal, but at some point, it stopped working.  I removed it because it was breaking ChromeVox on the dialog (see https://codereview.chromium.org/2787033002).  This was a quick fix to a bigger problem, but the modality of the dialog should also get fixed now that the initial MD-OOBE release is done.

[This issue was originally reported here: https://bugs.chromium.org/p/chromium/issues/detail?id=690726#c5]