Add support for loading API keys from Info.plist on iOS and macOS.

google_apis/google_api_keys_mac.h

Index: google_apis/google_api_keys_mac.h
diff --git a/google_apis/google_api_keys_mac.h b/google_apis/google_api_keys_mac.h
new file mode 100644
index 0000000000000000000000000000000000000000..6ee268eaae3980d7f1032d0a864a168ef89fade4
--- /dev/null
+++ b/google_apis/google_api_keys_mac.h
@@ -0,0 +1,16 @@
+// Copyright 2016 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 GOOGLE_API_KEYS_MAC_H_
+#define GOOGLE_API_KEYS_MAC_H_
+
+#include <string>
+
+namespace google_apis {
+
+std::string GetAPIKeyFromInfoPlist(const std::string& key_name);
+
+}  // namespace google_apis
+
+#endif  // GOOGLE_API_KEYS_MAC_H_