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

CookieIOS.CookieCountOnForegrounding
CookieIOS.CookieWrittenCount

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/cookie_store_ios.mm and ios/chrome/app/application_delegate/app_state.mm, marq@ as OWNERS of both directories can you take a look.