[prefs] Add CreateAutomationPrefService for browser automation

This CL adds a variant of CreateIncognitoPrefServiceSyncable intended
for OTR profiles created via browser automation (the CDP method
"Target.createBrowserContext").

Some browser UI, DevTools in particular, can be automated via CDP.
Syncing preferences across CDP created browser contexts defeats their
purpose.

Change-Id: Ibcd5542d8515831621b5866b160ea505a815030d
Fixed: 382623724
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6086128
Reviewed-by: Mihai Sardarescu <[email protected]>
Commit-Queue: Simon Zünd <[email protected]>
Code-Coverage: [email protected] <[email protected]>
Reviewed-by: Dominic Battré <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1395452}
diff --git a/chrome/browser/prefs/pref_service_syncable_util.cc b/chrome/browser/prefs/pref_service_syncable_util.cc
index 8c89312..9efa136e4 100644
--- a/chrome/browser/prefs/pref_service_syncable_util.cc
+++ b/chrome/browser/prefs/pref_service_syncable_util.cc
@@ -25,3 +25,10 @@
       incognito_extension_pref_store,
       prefs::GetIncognitoPersistentPrefsAllowlist());
 }
+
+std::unique_ptr<sync_preferences::PrefServiceSyncable>
+CreateAutomationPrefService(sync_preferences::PrefServiceSyncable* pref_service,
+                            PrefStore* incognito_extension_pref_store) {
+  return pref_service->CreateIncognitoPrefService(
+      incognito_extension_pref_store, /* persistent_pref_names */ {});
+}