Add capability to restrict variations studies by country.
This CL adds two new optional filter parameters to studies, "country"
and "exclude_country", that allow studies to be enabled or disabled in
specific countries.
The country code is supplied by the variations server as part of the
variations seed response. Chrome uses the latest country code received
for filtering session-consistency studies. For permanent-consistency
field trials, Chrome uses the first country code received for the
current Chrome version.
BUG=477039
Review URL: https://codereview.chromium.org/
1122083004
Cr-Commit-Position: refs/heads/master@{#328879}