[Sync] Add intelligent re-encryption support.

chrome/browser/sync/util/cryptographer.h

Index: chrome/browser/sync/util/cryptographer.h
diff --git a/chrome/browser/sync/util/cryptographer.h b/chrome/browser/sync/util/cryptographer.h
index 992c443900b17b1236bf4813cadac76cd9511fa3..1ae62ef63dd1aead5427ff21c45358ca9e2577bb 100644
--- a/chrome/browser/sync/util/cryptographer.h
+++ b/chrome/browser/sync/util/cryptographer.h
@@ -103,9 +103,12 @@ class Cryptographer {
   // key.
   bool CanDecryptUsingDefaultKey(const sync_pb::EncryptedData& encrypted) const;
 
-  // Encrypts |message| into |encrypted|. Returns true unless encryption fails.
-  // Note that encryption will fail if |message| isn't valid (eg. a required
-  // field isn't set).
+  // Encrypts |message| into |encrypted|. Does not overwrite |encrypted| if
+  // |message| already matches the decrypted data within |encrypted| and
+  // |encrypted| was encrypted with the current default key. This avoids
+  // unnecessarily modifying |encrypted| if the change had no practical effect.
+  // Returns true unless encryption fails or |message| isn't valid (e.g. a
+  // required field isn't set).
   bool Encrypt(const ::google::protobuf::MessageLite& message,
                sync_pb::EncryptedData* encrypted) const;
 
@@ -190,6 +193,10 @@ class Cryptographer {
   FRIEND_TEST_ALL_PREFIXES(CryptographerTest, PackUnpack);
   typedef std::map<std::string, linked_ptr<const Nigori> > NigoriMap;
 
+  bool EncryptImpl(const std::string& serialized,
+                   const NigoriMap::value_type* nigori,
+                   sync_pb::EncryptedData* encrypted) const;
+
   // Merges the given set of encrypted types with the existing set and emits a
   // notification if necessary.
   void MergeEncryptedTypes(const syncable::ModelTypeSet& encrypted_types);