Add is_chromecast to GN build.

chromecast/media/cma/backend/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("backend") {
   sources = [
     "audio_pipeline_device_default.cc",
     "audio_pipeline_device_default.h",
     "media_clock_device_default.cc",
     "media_clock_device_default.h",
     "media_component_device_default.cc",
     "media_component_device_default.h",
     "media_pipeline_backend_default.cc",
     "media_pipeline_backend_default.h",
     "video_pipeline_device_default.cc",
     "video_pipeline_device_default.h",
   ]
 
+  # This target cannot depend on //media. Include these headers directly.
+  sources += [
+    "//media/base/media_export.h",
+    "//media/base/timestamp_constants.h",
+  ]
+
   public_deps = [
     "//chromecast/public",
     "//chromecast/public/media",
   ]
 
   deps = [
     "//base",
-    "//chromecast/media/cma/base",
-    "//media",
   ]
 
   configs += [ "//chromecast:config" ]
 }