[CompactMode] Remove compact mode

Deprecating compact mode since it was an experimental prototype we are
no longer pursuing.

The associated pref will be deleted in 1 year per guidance in
chrome/browser/prefs/README.md

Change-Id: Ie27861a6ccc27afc8be406ec2fdf380e86286309
Bug: 355233862, 373474251
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6158254
Reviewed-by: Eshwar Stalin <[email protected]>
Reviewed-by: Marc Treib <[email protected]>
Commit-Queue: Darryl James <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1404266}
diff --git a/chrome/browser/prefs/browser_prefs.cc b/chrome/browser/prefs/browser_prefs.cc
index a4c76915..0a9c6a3 100644
--- a/chrome/browser/prefs/browser_prefs.cc
+++ b/chrome/browser/prefs/browser_prefs.cc
@@ -1181,6 +1181,9 @@
 inline constexpr char kPageContentCollectionEnabled[] =
     "page_content_collection.enabled";
 
+// Deprecated 01/2025.
+inline constexpr char kCompactModeEnabled[] = "compact_mode";
+
 // Register local state used only for migration (clearing or moving to a new
 // key).
 void RegisterLocalStatePrefsForMigration(PrefRegistrySimple* registry) {
@@ -1683,6 +1686,9 @@
 
   // Deprecated 12/2024.
   registry->RegisterBooleanPref(kPageContentCollectionEnabled, false);
+
+  // Deprecated 01/2025.
+  registry->RegisterBooleanPref(kCompactModeEnabled, false);
 }
 
 }  // namespace
@@ -2449,8 +2455,6 @@
   registry->RegisterIntegerPref(prefs::kLensOverlayStartCount, 0);
 
   registry->RegisterDictionaryPref(prefs::kReportingEndpoints);
-
-  registry->RegisterBooleanPref(prefs::kCompactModeEnabled, false);
 }
 
 void RegisterUserProfilePrefs(user_prefs::PrefRegistrySyncable* registry) {
@@ -3025,6 +3029,9 @@
   // Added 12/2024.
   profile_prefs->ClearPref(kPageContentCollectionEnabled);
 
+  // Added 01/2025.
+  profile_prefs->ClearPref(kCompactModeEnabled);
+
   // Please don't delete the following line. It is used by PRESUBMIT.py.
   // END_MIGRATE_OBSOLETE_PROFILE_PREFS