This is not currently exposed as a user-settable preference through
`tailscale up` or `tailscale set`. Instead, the preference is set when
turning the web client on and off via localapi. In a subsequent commit,
the pref will be used to automatically start the web client on startup
when appropriate.
Updates tailscale/corp#14335
Signed-off-by: Will Norris <will@tailscale.com>