[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..d5af0a7be3412624702aab7cb6cdec1b977cab14
--- /dev/null
+++ b/components/cast_channel/BUILD.gn
@@ -0,0 +1,25 @@
+# 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",
+  ]
+}
+
+source_set("unit_tests") {
+  testonly = true
+  sources = [
+    "cast_channel_enum_unittest.cc",
+  ]
+  deps = [
+    ":cast_channel",
+    "//base",
+    "//testing/gtest",
+  ]
+}