If any single STUN or TURN server is down, invalid uri/url is provided or something went wrong; chrome fails to skip that server; and unable to use next available STUN/TURN server from ice-servers list.
If TURN server is also linked; then chrome tries TURN earlier/sooner/quickly for ICE trickling/gathering process i.e. TURN is given high priority comparing STUN!
If a STUN server is down; however TURN server works; then WebRTC connection will be, obviously, established (according to TURN server availability and valid candidates generation) because TURN will be prompted first for candidates lookup process.
If all STUN servers are working fine; however, any single TURN server is down; then all STUN servers will be skipped and chrome will NEVER try those STUN servers for ICE lookup process!