[cast_channel] Make cast_channel related files not depend on "cast_channel.h"

components/cast_channel/BUILD.gn

Index: components/cast_channel/BUILD.gn
diff --git a/components/cast_channel/BUILD.gn b/components/cast_channel/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..ef44a6e87358ea7a760ca231e2da180290d484b9
--- /dev/null
+++ b/components/cast_channel/BUILD.gn
@@ -0,0 +1,13 @@
+# Copyright 2017 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.
+
+static_library("cast_channel") {
+  sources = [
+    "cast_channel_enum.cc",
+    "cast_channel_enum.h",
+  ]
+  deps = [
+    "//base",
+  ]
+}