Disable "Ok Google" hotwording in open source builds by default.
The compile-time flag "enable_hotwording" is now tied to
branding=Chrome (false by default unless making a Google Chrome build).
Note: Chromium will no longer download/install the Hotword Shared
Module, and will automatically remove the Hotword Shared Module on
startup if it was previously installed. To keep this functionality, add
"enable_hotwording=1" to GYP_DEFINES.