Decompose //extensions/browser/BUILD.gn

extensions/browser/api/cast_channel/BUILD.gn

+# Copyright 2015 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.
+source_set("cast_channel") {
+  sources = [
+    "cast_auth_util.cc",
+    "cast_auth_util.h",
+    "cast_channel_api.cc",
+    "cast_channel_api.h",
+    "cast_framer.cc",
+    "cast_framer.h",
+    "cast_message_util.cc",
+    "cast_message_util.h",
+    "cast_socket.cc",
+    "cast_socket.h",
+    "cast_transport.cc",
+    "cast_transport.h",
+    "keep_alive_delegate.cc",
+    "keep_alive_delegate.h",
+    "logger.cc",
+    "logger.h",
+    "logger_util.cc",
+    "logger_util.h",
+  ]
+
+  deps = [
+    "//content/public/browser",
+    "//extensions/common/api/cast_channel:cast_channel_proto",
+    "//skia",
+  ]
+}