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

MemoryWarning.OccurredDuringCardStackSession

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 is used from ios/chrome/browser/ui/stack_view/stack_view_controller.mm, kkhorimoto@ as OWNERS can you take a look.