[network] Change signature of empty protocol message handler
[network] Change signature of empty protocol message handler

This is pre-work to make back-end protocol changes landable.

Bug: chromium:1032063
Change-Id: I0d12dcd2175f1d00913656075cb839358c2c6cc0
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/1971413
Commit-Queue: Sigurd Schneider <f04d242e109628a27a0f75fd49a87722bcf291fd@chromium.org>
Reviewed-by: Jan Scheffler <8abd9e3d93041d096604cb2a1b3070dd7075ba7b@chromium.org>

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