[js] Use JavaScript 2020 in more configs (#522)
[js] Use JavaScript 2020 in more configs (#522)

Stop specifying the "--language_out" flag to Closure Compiler, since
its recent versions complain about using it together with the
"--browser_featureset_year" flag. This commit leaves only
"--browser_featureset_year" used in our code.

Also start using the 2020 version of JavaScript (ECMAScript) in
more build configurations, and only stick to the old 2012 version in the
"App+PNaCl" build. Both of Emscripten and WebUSB require a fresh (at
least from 2020) version of Chrome.
googlechromelabs/chromeos_smart_card_connector,GoogleChromeLabs/chromeos_smart_card_connector,GoogleChromeLabs/chromeos_smart_card_connector,googlechromelabs/chromeos_smart_card_connector,googlechromelabs/chromeos_smart_card_connector,googlechromelabs/chromeos_smart_card_connector,googlechromelabs/chromeos_smart_card_connector,GoogleChromeLabs/chromeos_smart_card_connector,GoogleChromeLabs/chromeos_smart_card_connector,GoogleChromeLabs/chromeos_smart_card_connector,GoogleChromeLabs/chromeos_smart_card_connector,googlechromelabs/chromeos_smart_card_connector