Add externs in DisplayUnitInfo.
Add externs in DisplayUnitInfo.

- mirroringDestinationIds
- availableDisplayZoomFactors
- displayZoomFactor

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=214693334

google/closure-compiler,monetate/closure-compiler,ChadKillingsworth/closure-compiler,monetate/closure-compiler,google/closure-compiler,google/closure-compiler,nawawi/closure-compiler,google/closure-compiler,vobruba-martin/closure-compiler,Yannic/closure-compiler,tiobe/closure-compiler,ChadKillingsworth/closure-compiler,Yannic/closure-compiler,shantanusharma/closure-compiler,shantanusharma/closure-compiler,nawawi/closure-compiler,ChadKillingsworth/closure-compiler,nawawi/closure-compiler,Yannic/closure-compiler,tiobe/closure-compiler,Yannic/closure-compiler,vobruba-martin/closure-compiler,shantanusharma/closure-compiler,vobruba-martin/closure-compiler,monetate/closure-compiler,nawawi/closure-compiler,vobruba-martin/closure-compiler,tiobe/closure-compiler,monetate/closure-compiler,tiobe/closure-compiler,ChadKillingsworth/closure-compiler,shantanusharma/closure-compiler