Clean up reading list and side panel feature flag code.

-Remove usage of read later flag on desktop.
-Remove unused read later new badge promo flag and use.
-Remove read later add from dialog flag and use (not used with side
panel).
-Remove side panel flag as the side panel has launched.
-Remove side panel flag from chrome labs.
-Remove code for stand-alone reading list bubble since it is not used
with the side panel.
-Remove kShowReadingListInBookmarkBar pref according to instructions at
chrome/browser/prefs/README.md

Bug: 1141224
Change-Id: I8408926c217df357b80be786a3d590d9cac6b4ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3557809
Reviewed-by: Dana Fried <[email protected]>
Reviewed-by: Alex Gough <[email protected]>
Reviewed-by: Caitlin Fischer <[email protected]>
Reviewed-by: Gabriel Charette <[email protected]>
Reviewed-by: Olivier Robin <[email protected]>
Reviewed-by: Mikel Astiz <[email protected]>
Reviewed-by: David Roger <[email protected]>
Commit-Queue: Caroline Rising <[email protected]>
Cr-Commit-Position: refs/heads/main@{#987938}
diff --git a/chrome/browser/prefs/browser_prefs.cc b/chrome/browser/prefs/browser_prefs.cc
index d9d6b4c..9de8e09 100644
--- a/chrome/browser/prefs/browser_prefs.cc
+++ b/chrome/browser/prefs/browser_prefs.cc
@@ -743,6 +743,7 @@
 #endif
 const char kStabilityExtensionRendererLaunchCount[] =
     "user_experience_metrics.stability.extension_renderer_launch_count";
+const char kShowReadingListInBookmarkBar[] = "bookmark_bar.show_reading_list";
 
 // Register local state used only for migration (clearing or moving to a new
 // key).
@@ -967,6 +968,8 @@
 #if BUILDFLAG(IS_CHROMEOS_ASH)
   registry->RegisterIntegerPref(kPhoneHubCameraRollPendingStatePrefName, 0);
 #endif  // BUILDFLAG(IS_CHROMEOS_ASH)
+
+  registry->RegisterBooleanPref(kShowReadingListInBookmarkBar, true);
 }
 
 }  // namespace
@@ -1910,6 +1913,9 @@
   crostini::RemoveDuplicateContainerEntries(profile_prefs);
 #endif
 
+  // Added 03/2022
+  profile_prefs->ClearPref(kShowReadingListInBookmarkBar);
+
   // Please don't delete the following line. It is used by PRESUBMIT.py.
   // END_MIGRATE_OBSOLETE_PROFILE_PREFS