Add macaroone implementation from
    https://weave.googlesource.com/weave/libuweave

BUG:25934771
Change-Id: Iba87e709a51006f3e966b9942a375d1c31b2f17d
Reviewed-on: https://weave-review.googlesource.com/1804
Reviewed-by: Alex Vakulenko <avakulenko@google.com>
diff --git a/third_party/libuweave/src/macaroon_context.c b/third_party/libuweave/src/macaroon_context.c
new file mode 100644
index 0000000..7477784
--- /dev/null
+++ b/third_party/libuweave/src/macaroon_context.c
@@ -0,0 +1,22 @@
+// Copyright 2015 The Weave Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "src/macaroon_context.h"
+
+#include "src/macaroon_caveat.h"
+
+bool uw_macaroon_context_get_(UwMacaroonCaveatType type,
+                              const uint8_t** context, size_t* context_len) {
+  if (type != kUwMacaroonCaveatTypeSessionIdentifier) {
+    *context = NULL;
+    *context_len = 0;
+  }
+
+  // TODO(bozhu): Waiting for a proper way to obtain the session identifier.
+  // Have we already implemented something related to session identifiers?
+  *context = NULL;
+  *context_len = 0;
+
+  return true;
+}