Since you saw in the source code, did you read the comment around line
90 from:
http://src.chromium.org/svn/trunk/src/chrome/installer/util/google_update_settings.h
// Return a human readable modifier for the version string, e.g.
// the channel (dev, beta, stable). Returns true if this operation
succeeded,
// on success, channel contains one of "", "unknown", "dev" or
"beta" (unless
// it is a multi-install product, in which case it will return "m",
// "unknown-m", "dev-m", or "beta-m").
static bool GetChromeChannel(bool system_install, std::wstring*
channel);