Add comments on deprecated SameSite=Extended enum values.

We're keeping these enum values only for conversion of old DB
values and UMA metrics.

Bug: 953995
Change-Id: I98681a90c81ee3d6d27dc1e8ea0100334cfc0897
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1890537Reviewed-by: Lily Chen <chlily@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711082}

net/cookies/cookie_constants.h

@@ -64,7 +64,7 @@ enum class CookieSameSiteString {
   kLax = 3,
   kStrict = 4,
   kNone = 5,
-  kExtended = 6,
+  kExtended = 6,  // Deprecated, kept for metrics only.
 
   // Keep last, update if adding new value.
   kMaxValue = kExtended

net/extras/sqlite/sqlite_persistent_cookie_store.cc

@@ -480,6 +480,7 @@ enum DBCookieSameSite {
   kCookieSameSiteNoRestriction = 0,
   kCookieSameSiteLax = 1,
   kCookieSameSiteStrict = 2,
+  // Deprecated, mapped to kCookieSameSiteUnspecified.
   kCookieSameSiteExtended = 3
 };