Update endpoint URLs for the kids management API

Also make them configurable via a variation param.

BUG=678958

Review-Url: https://codereview.chromium.org/2655313002
Cr-Commit-Position: refs/heads/master@{#446647}
diff --git a/chrome/browser/BUILD.gn b/chrome/browser/BUILD.gn
index 33250439..443d291 100644
--- a/chrome/browser/BUILD.gn
+++ b/chrome/browser/BUILD.gn
@@ -3673,6 +3673,8 @@
       "supervised_user/child_accounts/child_account_service_factory.h",
       "supervised_user/child_accounts/family_info_fetcher.cc",
       "supervised_user/child_accounts/family_info_fetcher.h",
+      "supervised_user/child_accounts/kids_management_api.cc",
+      "supervised_user/child_accounts/kids_management_api.h",
       "supervised_user/child_accounts/permission_request_creator_apiary.cc",
       "supervised_user/child_accounts/permission_request_creator_apiary.h",
       "supervised_user/experimental/safe_search_url_reporter.cc",