[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 2c66bbea9d9344a91be6ed5a6bdc798cb1c39cd9..cb78e6c2ce39eb3e21fb736e2bf496d8498c45bd 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;