Chromium on Linux has several possible sources of proxy info: GNOME/KDE settings, command-line flags, and environment variables.
When Chromium detects that it is running in GNOME or KDE, it will automatically use the appropriate standard proxy settings. You can configure these proxy settings from the options dialog (the “Change proxy settings” button in the “Under the Hood” tab), which will launch the GNOME or KDE proxy settings applications, or by launching those applications directly.
For other desktop environments, Chromium's proxy settings can be configured using command-line flags or environment variables. These are documented on the man page (man google-chrome or man chromium-browser).