In addition, we should confirm that the failure reason metrics continue to report correct values after https://bugs.chromium.org/p/chromium/issues/detail?id=542862 is fixed. CL: https://codereview.chromium.org/1751183002/

Pertinent UMA from Dev on March 23, 2016. 
Number of users with Precache enabled: 50,440

https://uma.googleplex.com/p/chrome/histograms/?endDate=03-23-2016&dayCount=7&histograms=Net.ErrorCodesForMainFrame3&fixupData=true&uniqueUsers=true&showMax=true&filters=platform%2Ceq%2CA%2Cchannel%2Ceq%2C2%2Cfinch%2Cstgr_filter%2C2655433701%7C1061820383%7C1423453153%2Cisofficial%2Ceq%2CTrue&implicitFilters=isofficial


Number of users where Precache tries to start: 34,431
Number of users where the start is successful(Failure Reason == 0): 17,039
https://uma.googleplex.com/p/chrome/histograms/?endDate=03-23-2016&dayCount=7&histograms=Precache.Fetch.FailureReasons&fixupData=true&uniqueUsers=true&showMax=true&filters=platform%2Ceq%2CA%2Cchannel%2Ceq%2C2%2Cfinch%2Cstgr_filter%2C2655433701%7C1061820383%7C1423453153%2Cisofficial%2Ceq%2CTrue&implicitFilters=isofficial

Users with precached fetch bytes: 15,945

https://uma.googleplex.com/p/chrome/histograms/?endDate=03-23-2016&dayCount=7&histograms=Precache.Fetch.ResponseBytes.Total&fixupData=true&uniqueUsers=true&showMax=true&filters=platform%2Ceq%2CA%2Cchannel%2Ceq%2C2%2Cfinch%2Cstgr_filter%2C2655433701%7C1061820383%7C1423453153%2Cisofficial%2Ceq%2CTrue&implicitFilters=isofficial

Perhaps we are misunderstanding the first UMA above. If I use a different UMA and then filter by Precache, then the number is in agreement with the other Precache metrics. For example, using  Net.DailyContentLength gives 33,376 users with Precache enabled.

https://uma.googleplex.com/p/chrome/histograms/?endDate=03-23-2016&dayCount=7&histograms=Net.DailyContentLength%2CNet.ErrorCodesForMainFrame3&fixupData=true&uniqueUsers=true&showMax=true&filters=platform%2Ceq%2CA%2Cchannel%2Ceq%2C2%2Cfinch%2Cstgr_filter%2C2655433701%7C1061820383%7C1423453153%2Cisofficial%2Ceq%2CTrue&implicitFilters=isofficial

Precache.Fetch.FailureReasons and Net.ErrorCodesForMainFrame3 get logged at different locations and with very different frequencies, so the number of users reporting these UMAs are not expected to match up.

There are multiple reasons why a UMA being recorded less frequently will see a smaller number of users, such as loss of connectivity, device restart, process being killed before UMA is sent across the wire, etc.

This appears to be a reporting issue. The number of users who save data due to precaching is around 50.5% of all users in the experiment (25,473). See:

https://uma.googleplex.com/p/chrome/histograms/?endDate=03-23-2016&dayCount=7&histograms=Precache.Saved&fixupData=true&uniqueUsers=true&showMax=true&filters=platform%2Ceq%2CA%2Cchannel%2Ceq%2C2%2Cfinch%2Cstgr_filter%2C2655433701%7C1061820383%7C1423453153%2Cisofficial%2Ceq%2CTrue&implicitFilters=isofficial

I believe the reporting issue is that UMA is not reliably reported each time precache runs, because the process often will complete and shutdown before reporting can take place. See https://bugs.chromium.org/p/chromium/issues/detail?id=583440

I guess this issue is obsolete now.
We have more understanding of these UMA and/or fixed a bunch of issues around it.