Revert of [cast_channel] Move cast_channel related files from //extensions to //components

extensions/common/api/cast_channel/BUILD.gn

Index: extensions/common/api/cast_channel/BUILD.gn
diff --git a/extensions/common/api/cast_channel/BUILD.gn b/extensions/common/api/cast_channel/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..e7d76b388dfdfdf044d24154d8f512cff860d5e3
--- /dev/null
+++ b/extensions/common/api/cast_channel/BUILD.gn
@@ -0,0 +1,13 @@
+# 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.
+
+import("//third_party/protobuf/proto_library.gni")
+
+proto_library("cast_channel_proto") {
+  sources = [
+    "authority_keys.proto",
+    "cast_channel.proto",
+    "logging.proto",
+  ]
+}