On Linux, we obtain mmaps for all processes from the browser process:
https://cs.chromium.org/chromium/src/services/resource_coordinator/memory_instrumentation/queued_request_dispatcher.cc?l=253

The logic for filling in debug_id attempts to only do so for the mmap that contains the &ParseSmapsHeader function:
https://cs.chromium.org/chromium/src/services/resource_coordinator/public/cpp/memory_instrumentation/os_metrics_linux.cc?type=cs&q=os_metrics_linux.cc&sq=package:chromium&g=0&l=106

Unfortunately, this assumes that the mmap is for the current process. 

ssid: Why is debug_id logic scoped to only the Chrome image. Is this a performance optimization?