We have a bunch of things like this, in various embedders:
https://cs.chromium.org/chromium/src/content/shell/browser/shell_net_log.cc
https://cs.chromium.org/chromium/src/headless/lib/browser/headless_net_log.cc
which basically differ only in the stuff they stuff inside the thing in
GetConstants.
(Chrome proper's version is more complicated, but it has all this stuff,
too).

Now, long term the NetLog should be owned by the Network Service (it's only
kinda doing this sometimes now, but fixing that is messy),
and so it would be nice if in the network service world there was a
shared way of getting the functionality those classes provide,
in particular --log-net-log and sticking in of some customized constants
into thelog preamble.

The network service process cannot do this alone, however, since it will be
sandboxed, and will therefore be unable to open an arbitrary
file requested by user --- so the setup code in browser process in content/
will need to open the file if the flag is there, ask the embedder for
constants it wants (maybe via a method on ContentBrowserClient, perhaps
with API similar to GetPlatformConstants here:

https://chromium-review.googlesource.com/c/chromium/src/+/1016669/35/components/net_log/chrome_net_log.cc#71),

and pass it over to the network service over a mojo call, which will do
something that will probably replace this:
https://cs.chromium.org/chromium/src/services/network/mojo_net_log.cc?rcl=e65d6b85e45f22eae7b664457b645d291a7743de&l=24
(using CreateUnboundedPreExisting instead of CreateUnbounded).