Move files from platform/api to platform/impl

Yuri commented on my open CL (
https://chromium-review.googlesource.com/c/openscreen/+/1791373) that 2
files currently in platform/api should be moved to platform/impl:
- platform/api/network_waiter.h
- platform/api/socket_handle.h

This CL accomplishes that, so that the attached CL can remain small

Change-Id: I7aa1b48e4c6cdfbe512a9361433b81fc8421b044
Reviewed-on: https://chromium-review.googlesource.com/c/openscreen/+/1809484
Reviewed-by: Jordan Bayles <jophba@chromium.org>
Commit-Queue: Ryan Keane <rwkeane@google.com>
diff --git a/platform/impl/socket_handle.h b/platform/impl/socket_handle.h
new file mode 100644
index 0000000..9aea5f1
--- /dev/null
+++ b/platform/impl/socket_handle.h
@@ -0,0 +1,18 @@
+// Copyright 2019 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 PLATFORM_IMPL_SOCKET_HANDLE_H_
+#define PLATFORM_IMPL_SOCKET_HANDLE_H_
+
+namespace openscreen {
+namespace platform {
+
+// A SocketHandle is the handle used to access a Socket by the underlying
+// platform.
+struct SocketHandle;
+
+}  // namespace platform
+}  // namespace openscreen
+
+#endif  // PLATFORM_IMPL_SOCKET_HANDLE_H_