[CrOS Settings] Add reminder to update enums.xml

Otherwise one might only find out when
OsSettingsManagerTest.Initialization fails during the submit process.

Change-Id: I86f2135f3aca46fdad6cab4be7337de211af625d
Bug: none
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2542732Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Auto-Submit: Harry Cutts <hcutts@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828196}

chrome/browser/ui/webui/settings/chromeos/constants/setting.mojom

@@ -5,7 +5,8 @@
 module chromeos.settings.mojom;
 
 // Enumeration of each setting in the Chrome OS Settings app. Numerical values
-// are used for metrics; do not change or reuse values.
+// are used for metrics; do not change or reuse values. Add new values to the
+// OsSetting enum in tools/metrics/histograms/enums.xml.
 enum Setting {
   // Network section.
   kConfigureEthernet = 0,