Add a do_not_sync pref to ExtensionPrefs.

chrome/common/extensions/sync_helper.h

Index: chrome/common/extensions/sync_helper.h
diff --git a/chrome/common/extensions/sync_helper.h b/chrome/common/extensions/sync_helper.h
index 3a91bcb11da64e83b94767320bbc1b8194a422bc..db8d913f9a0fe1809011790875a172ca407a9cc6 100644
--- a/chrome/common/extensions/sync_helper.h
+++ b/chrome/common/extensions/sync_helper.h
@@ -11,6 +11,11 @@ class Extension;
 
 namespace sync_helper {
 
+// NOTE: The check in the functions here only considers the data in extension
+// itself, not the environment it is in. To determine whether an extension
+// should be synced, you probably want to use util::ShoudSyncExtension and
+// util::ShouldSyncApp.
+
 // Returns true if the extension should be synced.
 bool IsSyncable(const Extension* extension);