Make super MAC optional.

chrome/browser/prefs/pref_hash_store_impl.cc

Index: chrome/browser/prefs/pref_hash_store_impl.cc
diff --git a/chrome/browser/prefs/pref_hash_store_impl.cc b/chrome/browser/prefs/pref_hash_store_impl.cc
index a82d449f4e40ff4618ca6c998a3c563ffafd56d5..4f6f6d6754beddc52616934a6156f4cf9dd425fb 100644
--- a/chrome/browser/prefs/pref_hash_store_impl.cc
+++ b/chrome/browser/prefs/pref_hash_store_impl.cc
@@ -59,11 +59,15 @@ class PrefHashStoreImpl::PrefHashStoreTransactionImpl
 
 PrefHashStoreImpl::PrefHashStoreImpl(const std::string& seed,
                                      const std::string& device_id,
-                                     scoped_ptr<HashStoreContents> contents)
+                                     scoped_ptr<HashStoreContents> contents,
+                                     bool use_super_mac)
     : pref_hash_calculator_(seed, device_id),
       contents_(contents.Pass()),
       initial_hashes_dictionary_trusted_(
-          IsHashDictionaryTrusted(pref_hash_calculator_, *contents_)),
+          use_super_mac
+              ? IsHashDictionaryTrusted(pref_hash_calculator_, *contents_)
+              : false),
+      use_super_mac_(use_super_mac),
       has_pending_write_(false) {
   DCHECK(contents_);
   UMA_HISTOGRAM_BOOLEAN("Settings.HashesDictionaryTrusted",
@@ -97,11 +101,13 @@ PrefHashStoreImpl::PrefHashStoreTransactionImpl::
   // Update the super MAC if and only if the hashes dictionary has been
   // modified in this transaction.
   if (has_changed_) {
-    // Get the dictionary of hashes (or NULL if it doesn't exist).
-    const base::DictionaryValue* hashes_dict = outer_->contents_->GetContents();
-    outer_->contents_->SetSuperMac(outer_->pref_hash_calculator_.Calculate(
-        outer_->contents_->hash_store_id(), hashes_dict));
-
+    if (outer_->use_super_mac_) {
+      // Get the dictionary of hashes (or NULL if it doesn't exist).
+      const base::DictionaryValue* hashes_dict =
+          outer_->contents_->GetContents();
+      outer_->contents_->SetSuperMac(outer_->pref_hash_calculator_.Calculate(
+          outer_->contents_->hash_store_id(), hashes_dict));
+    }
     outer_->has_pending_write_ = true;
   }