Add is_chromecast to GN build.

chromecast/media/cma/backend/BUILD.gn

Index: chromecast/media/cma/backend/BUILD.gn
diff --git a/chromecast/media/cma/backend/BUILD.gn b/chromecast/media/cma/backend/BUILD.gn
index c186404b5599f07b0cd141a0e624402fb0863e17..7507b7d6ea0e7c6c6d90e303e58e1da6186a30bb 100644
--- a/chromecast/media/cma/backend/BUILD.gn
+++ b/chromecast/media/cma/backend/BUILD.gn
@@ -16,6 +16,12 @@ source_set("backend") {
     "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",
@@ -23,8 +29,6 @@ source_set("backend") {
 
   deps = [
     "//base",
-    "//chromecast/media/cma/base",
-    "//media",
   ]
 
   configs += [ "//chromecast:config" ]