Measure actual KB in NetworkService.MaskedDomainList.Size

This value is measured in two places because the proto is deserialized
in different processes in depending on a feature flag. One of those
places (for flatbuffer) divided by 1024, but the the `MdlSize()`
function also divides by 1024.

OBSOLETE_HISTOGRAM[NetworkService.MaskedDomainList.Size]=Replaced by NetworkService.MaskedDomainList.Size2

Change-Id: I32777db72f262b32cd530e074edc7628ad460d30
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6340533
Reviewed-by: Sorin Jianu <sorin@chromium.org>
Reviewed-by: Matthew Wang <matthewmwang@chromium.org>
Commit-Queue: Dustin Mitchell <djmitche@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1432877}