Add comment to CachedFeatureFlags#sNonDynamicPrefKeys.

Bug: 1012975
Change-Id: Iafb25a6fcc6a003266c8cba49a6db3b5aa477c28
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2044761
Auto-Submit: Henrique Nakashima <hnakashima@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742431}

chrome/android/java/src/org/chromium/chrome/browser/flags/CachedFeatureFlags.java

@@ -70,6 +70,12 @@ public class CachedFeatureFlags {
         }
     };
 
+    /**
+     * Non-dynamic preference keys used historically for specific features.
+     *
+     * Do not add new values to this list. To add a new cached feature flag, just follow the
+     * instructions in the class javadoc.
+     */
     private static final Map<String, String> sNonDynamicPrefKeys = new HashMap<String, String>() {
         {
             put(ChromeFeatureList.SERVICE_MANAGER_FOR_DOWNLOAD,