[Sync::USS] Introduce about flag for USS bookmark sync

This allows more easy testing of the feature under development/
experimentation.

Bug: 516866
Change-Id: I8418b9c61ad42214491c8b97edfcfe71f879b14f
Reviewed-on: https://chromium-review.googlesource.com/1146818
Reviewed-by: Mikel Astiz <[email protected]>
Commit-Queue: Mohamed Amir Yosef <[email protected]>
Cr-Commit-Position: refs/heads/master@{#577200}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 6e32a64..43c476b 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -459,6 +459,9 @@
 extern const char kEnableSyncUserConsentSeparateTypeName[];
 extern const char kEnableSyncUserConsentSeparateTypeDescription[];
 
+extern const char kEnableSyncUSSBookmarksName[];
+extern const char kEnableSyncUSSBookmarksDescription[];
+
 extern const char kEnableSyncUSSSessionsName[];
 extern const char kEnableSyncUSSSessionsDescription[];