[Adaptation] Move Resource to api/ folder.

This is a prerequisite to implementing it externally.

Bug: webrtc:11525
Change-Id: I9cb3b4418396485d3eb9f25cafa51cbff6db7817
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/176401
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Evan Shrubsole <eshr@google.com>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31418}
diff --git a/api/adaptation/resource.cc b/api/adaptation/resource.cc
new file mode 100644
index 0000000..0a9c83a
--- /dev/null
+++ b/api/adaptation/resource.cc
@@ -0,0 +1,30 @@
+/*
+ *  Copyright 2019 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "api/adaptation/resource.h"
+
+namespace webrtc {
+
+const char* ResourceUsageStateToString(ResourceUsageState usage_state) {
+  switch (usage_state) {
+    case ResourceUsageState::kOveruse:
+      return "kOveruse";
+    case ResourceUsageState::kUnderuse:
+      return "kUnderuse";
+  }
+}
+
+ResourceListener::~ResourceListener() {}
+
+Resource::Resource() {}
+
+Resource::~Resource() {}
+
+}  // namespace webrtc