mini_installer is just a packer which packs the following files:
1. CHROME.PACKED.7z
2. setup.exe
These files should be present in your BuildFolder. CHROME.PACKED.7z packs Chrome.7z which includes your Chromium files and folders.
mini_installer will extract those two files to a temporary directory and then execute setup.exe. For instance if mini_installer.exe was executed with --system-level argument, it will pass those arguments to setup.exe:
"C:\Users\Username\AppData\Local\Temp\CWB_341A7.tmp\setup.exe" --install-archive="C:\Users\Username\AppData\Local\Temp\CWB_341A7.tmp\CHROME.PACKED.7Z" --system-level
That temporary folder name should be different for Chromium and Chrome cause we modified our fork to use CWB prefix
So, it's setup.exe which is responsible for the actual installation and uninstallation process. When you uninstall your Chromium fork setup.exe will be executed with the following arguments:
C:\Program Files (x86)\YourChromium\Application\66.0.3359.139\Installer\setup.exe --uninstall --system-level
Note: if you pass --system-level then it will be installed for all the users.
The arguments you pass to mini_installer will be stored in registry so the same argument will be passed to setup.exe when you uninstall it. It should typically be stored here:
HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\CompanyName\Update\ClientState\{Your-Chromium-fork-GUID} (for system-level installations)
The value of UninstallArguments and UninstallString will be read and used when uninstalling your Chromium fork.
Hope that helps
EDIT:
Just adding this info for anyone who might find this useful. There are various command line options for the installer which you can find here:
chrome\installer\util\master_preferences_constants.cc