Switch base::Value typemapping to be by value instead of by unique_ptr.

services/preferences/public/cpp/pref_store_client_mixin.cc

Index: services/preferences/public/cpp/pref_store_client_mixin.cc
diff --git a/services/preferences/public/cpp/pref_store_client_mixin.cc b/services/preferences/public/cpp/pref_store_client_mixin.cc
index 0387883265f206a0a0e34b1c0f981b34f923f239..c4bb88b41cc78408a3d6218caa10d1026868a8a8 100644
--- a/services/preferences/public/cpp/pref_store_client_mixin.cc
+++ b/services/preferences/public/cpp/pref_store_client_mixin.cc
@@ -59,10 +59,11 @@ PrefStoreClientMixin<BasePrefStore>::~PrefStoreClientMixin() = default;
 
 template <typename BasePrefStore>
 void PrefStoreClientMixin<BasePrefStore>::Init(
-    std::unique_ptr<base::DictionaryValue> initial_prefs,
+    const base::DictionaryValue* initial_prefs,
     bool initialized,
     mojom::PrefStoreObserverRequest observer_request) {
-  cached_prefs_ = std::move(initial_prefs);
+  if (initial_prefs)
+    cached_prefs_ = initial_prefs->CreateDeepCopy();
   observer_binding_.Bind(std::move(observer_request));
   if (initialized)
     OnInitializationCompleted(static_cast<bool>(cached_prefs_));
@@ -101,7 +102,7 @@ void PrefStoreClientMixin<BasePrefStore>::OnInitializationCompleted(
 template <typename BasePrefStore>
 void PrefStoreClientMixin<BasePrefStore>::OnPrefChanged(
     const std::string& key,
-    std::unique_ptr<base::Value> value) {
+    base::Optional<base::Value> value) {
   DCHECK(cached_prefs_);
   bool changed = false;
   if (!value) {  // Delete
@@ -110,12 +111,12 @@ void PrefStoreClientMixin<BasePrefStore>::OnPrefChanged(
   } else {
     const base::Value* prev;
     if (cached_prefs_->Get(key, &prev)) {
-      if (!prev->Equals(value.get())) {
-        cached_prefs_->Set(key, std::move(value));
+      if (*prev != *value) {
+        cached_prefs_->Set(key, value->CreateDeepCopy());
         changed = true;
       }
     } else {
-      cached_prefs_->Set(key, std::move(value));
+      cached_prefs_->Set(key, value->CreateDeepCopy());
       changed = true;
     }
   }