Missing flags for screen capture in recent Chrome Update
You cannot use those flags any longer and there is NO WAY to access them again short of rolling back your update. They were removed for security reasons. Now, you have to use a chrome plugin to do any desktop capture. Which is frustrating but the only way and it is fairly simple.
Here is an another SO question that should give you direction.
Firefox also now has screen/window/etc capture. For now, for similar reasons to Chrome, we've put access to the feature behind a whitelist (though an extension could extend the whitelist as well; we haven't prototyped that yet).
See http://mozilla.github.io/webrtc-landing/gum_test.html (In Firefox 33 or later - currently that's Beta; no attempt to polyfill was done there as it's an internal testing page).