[Sync] Add full history sync experiment

sync/internal_api/public/util/experiments.h

Index: sync/internal_api/public/util/experiments.h
diff --git a/sync/internal_api/public/util/experiments.h b/sync/internal_api/public/util/experiments.h
index 6e20a8e19b86e62432b107e8626fb80a98aab8cf..c13e56a6692d54caf142e7c27ad56ae18a84661f 100644
--- a/sync/internal_api/public/util/experiments.h
+++ b/sync/internal_api/public/util/experiments.h
@@ -12,17 +12,21 @@ namespace syncer {
 const char kKeystoreEncryptionTag[] = "keystore_encryption";
 const char kKeystoreEncryptionFlag[] = "sync-keystore-encryption";
 const char kAutofillCullingTag[] = "autofill_culling";
+const char kFullHistorySyncTag[] = "history_delete_directives";
+const char kFullHistorySyncFlag[] = "full-history-sync";
 
 // A structure to hold the enable status of experimental sync features.
 struct Experiments {
   Experiments() : sync_tab_favicons(false),
                   keystore_encryption(false),
-                  autofill_culling(false) {}
+                  autofill_culling(false),
+                  full_history_sync(false) {}
 
   bool Matches(const Experiments& rhs) {
     return (sync_tab_favicons == rhs.sync_tab_favicons &&
             keystore_encryption == rhs.keystore_encryption &&
-            autofill_culling == rhs.autofill_culling);
+            autofill_culling == rhs.autofill_culling &&
+            full_history_sync == rhs.full_history_sync);
   }
 
   // Enable syncing of favicons within tab sync (only has an effect if tab sync
@@ -34,6 +38,9 @@ struct Experiments {
 
   // Enable deletion of expired autofill entries (if autofill sync is enabled).
   bool autofill_culling;
+
+  // Enable full history sync (and history delete directives) for this client.
+  bool full_history_sync;
 };
 
 }  // namespace syncer