The following UMA histogram have no definition in either the public histograms.xml file (tools/metrics/histograms/histograms.xml) or the private one:

CookieStoreIOS.LossOnForegrounding
CookieStoreIOS.LossOnRead

The histogram thus cannot be seen on UMA using the histogram name, but are still visible using the correct hash. The histograms needs to be declared in tools/metrics/histograms/histograms.xml.

If the histogram is unused (likely since it is not declared), then the code to log the histogram should be removed, but the declaration in histograms.xml should be kept (see rationale in https://chromium.googlesource.com/chromium/src/+/master/tools/metrics/histograms/README.md#Deleting-Histograms).

Histograms are used from ios/net/cookies/system_cookie_util.mm, marq@ as OWNERS of ios/net, can you triage (I think neither of the other OWNERS works on iOS or net anymore).