Teach the GCM Driver how to decrypt incoming messages.

components/gcm_driver/crypto/gcm_message_cryptographer.h

Index: components/gcm_driver/crypto/gcm_message_cryptographer.h
diff --git a/components/gcm_driver/crypto/gcm_message_cryptographer.h b/components/gcm_driver/crypto/gcm_message_cryptographer.h
index 58c44a8b7cf217cf412421e9ef9ba01b49f4c6e8..e2b299e5860b45fd544e5463da1ff8dde271bfb4 100644
--- a/components/gcm_driver/crypto/gcm_message_cryptographer.h
+++ b/components/gcm_driver/crypto/gcm_message_cryptographer.h
@@ -32,6 +32,10 @@ namespace gcm {
 // messages provided that a cryptographically-strong random salt is used.
 class GCMMessageCryptographer {
  public:
+  // Salt size, in bytes, that will be used together with the key to create a
+  // unique content encryption key for a given message.
+  static const size_t kSaltSize;
+
   GCMMessageCryptographer();
   ~GCMMessageCryptographer();