suggestion from yfriedman:

You'd have to define a java bridge class that wraps your native model. 
Looks like the webui is rendered by this? https://cs.chromium.org/chromium/src/components/ukm/debug/ukm_debug_data_extractor.cc?type=cs&sq=package:chromium&l=38

Here's a fairly simple bridge class:
https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chrome/browser/consent_auditor/ConsentAuditorBridge.java?q=file:bridge+file:%5C.java&dr

So basically a java accessor to return that field. Sadly none of this seems to currently be exported to the java layer so it would require a bit of boilerplate