[Bug 1716139] [NEW] Remove "Ubuntu" identifier from User Agent strings

[spm2011]

Public bug reported:

The Chromium and Firefox builds add an extra Ubuntu identifier in their
user-agent strings. The inclusion of the OS name is unnecessary and
could contribute to user fingerprinting.

Other distros such as Fedora do not add their name to the distro browser
builds' UA as far as I can tell, and google-chrome UAs do not reveal OS
distro version info.

chromium-browser:
Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Ubuntu Chromium/60.0.3112.113 Chrome/60.0.3112.113 Safari/537.36

Change to:
Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chromium/60.0.3112.113 Chrome/60.0.3112.113 Safari/537.36


Firefox:
Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:55.0) Gecko/20100101 Firefox/55.0

Change to: Mozilla/5.0 (X11; Linux x86_64; rv:55.0) Gecko/20100101
Firefox/55.0

In Firefox the build ID can identify the package build anyway so it
isn't necessary to have the distro in the UA string.

** Affects: chromium-browser (Ubuntu)
     Importance: Undecided
         Status: New

** Affects: firefox (Ubuntu)
     Importance: Undecided
         Status: New

** Also affects: firefox (Ubuntu)
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1716139

Title:
  Remove "Ubuntu" identifier from User Agent strings

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