Introduce CryptAuthClient, a class capable of performing all CryptAuth APIs.

components/proximity_auth/cryptauth/cryptauth_access_token_fetcher.h

Index: components/proximity_auth/cryptauth/cryptauth_access_token_fetcher.h
diff --git a/components/proximity_auth/cryptauth/cryptauth_access_token_fetcher.h b/components/proximity_auth/cryptauth/cryptauth_access_token_fetcher.h
new file mode 100644
index 0000000000000000000000000000000000000000..04dbe2f5e43f0c9b3c18b8cd44ebd3570c97a9d6
--- /dev/null
+++ b/components/proximity_auth/cryptauth/cryptauth_access_token_fetcher.h
@@ -0,0 +1,25 @@
+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef COMPONENTS_PROXIMITY_AUTH_CRYPT_AUTH_ACCESS_TOKEN_FETCHER_H
+#define COMPONENTS_PROXIMITY_AUTH_CRYPT_AUTH_ACCESS_TOKEN_FETCHER_H
+
+namespace proximity_auth {
+
+// Simple interface for fetching the OAuth2 access token that authorizes
+// CryptAuth API calls.
+class CryptAuthAccessTokenFetcher {
+ public:
+  virtual ~CryptAuthAccessTokenFetcher() {}
+
+  // Fetches the access token asynchronously, invoking the callback upon
+  // completion. If the fetch fails, the callback will be invoked with an empty
+  // string.
+  typedef base::Callback<void(const std::string&)> AccessTokenCallback;
+  virtual void FetchAccessToken(const AccessTokenCallback& callback) = 0;
+};
+
+}  // namespace proximity_auth
+
+#endif  // COMPONENTS_PROXIMITY_AUTH_CRYPT_AUTH_ACCESS_TOKEN_FETCHER_H