Add BUILD.gn files for chromecast/media

chromecast/media/cma/base/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.
+
+component("base") {
+  sources = [
+    "balanced_media_task_runner_factory.cc",
+    "balanced_media_task_runner_factory.h",
+    "buffering_controller.cc",
+    "buffering_controller.h",
+    "buffering_defs.cc",
+    "buffering_defs.h",
+    "buffering_frame_provider.cc",
+    "buffering_frame_provider.h",
+    "buffering_state.cc",
+    "buffering_state.h",
+    "cma_logging.h",
+    "coded_frame_provider.cc",
+    "coded_frame_provider.h",
+    "decoder_buffer_adapter.cc",
+    "decoder_buffer_adapter.h",
+    "decoder_buffer_base.cc",
+    "decoder_buffer_base.h",
+    "media_task_runner.cc",
+    "media_task_runner.h",
+  ]
+  deps = [
+    "//chromecast/base",
+    "//base",
+    "//media",
+  ]
+}