As part of our long standing commitment to making the web safer to use, we will be conducting an experiment to validate our implementation of DNS-over-HTTPS (aka DoH) in Chrome 78.
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Our experiment will run on all supported platforms (with the exception of Linux and iOS) for a fraction of Chrome users. On Android 9 and above, if the user has specified a DNS-over-TLS provider in the private DNS settings, Chrome may use the associated DoH provider, and will fallback to the system private DNS upon error.
It depends on how Chrome determines the system's dns. If it simply looks up the OS's dns, then Chrome will not have a corresponding DoH server for your private pihole. But if it does a more exhaustive lookup, such as those on dns leak tests, and the upstream dns of your pihole has a DoH equivalent, then it may the DoH server directly.
It's not speculative. If the current dns has a corresponding DoH server then Chrome will automatically switch to DoH. How does Chrome determine if there's DoH server available? It's has a list of supported DoH servers:
Seems pretty clear to me. If you using one of the listed dns, then Chrome will use the DoH. If it's not on the list then Chrome won't make any changes.