Assigned to Zach because I don't know what some of these extensions do.

I think we want to get rid of PDF Viewer. 

The ones I'm less sure about are the Genius app, Mobile activation, and ChromeOS Help.

What's the motivation for pruning?
Haven't been able to track down why PDF Viewer is enabled, but concerned about a Chesterton's Fence scenario by removing them without knowing why they were added in the first place.

BTW, as per an offline discussion, seems that we still don't know whether there is a clear use case for the PDF Viewer in the sign-in profile.
However, one potential scenario where one can imagine it being used there is the SAML login flow with a third-party page embedding a PDF document.

And answering the question from comment 2: IIUC, the motivation for the pruning is 1) optimization, 2) security concerns.

@jdufault do we have metrics on how often these extensions are actually used? If we see that any are really and truly at zero usage, that'd be a good case for removing them.

I'm not aware of any metrics at the moment. This is something we should address as part of views-signin.