The answer was provided by Rob Rich at
https://chromium.woolyss.com/
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try building this target:
autoninja -C out\Default mini_installer.exe
That builds an Windows installer executable and a compressed 7z file in the output directory.
The installer is silent, so click it and wait a few seconds to maybe a minute on slower systems for the install to complete.
Alternatively, the 7z archive can be uncompressed and run from whatever directory if desired.
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It would be good to add this information to the Chromium documentation.
If you Google on the phrase "autoninja -C out\Default mini_installer.exe" you literally get zero results.