GN foreach should mark the list identifier as used.

media/BUILD.gn

 # Copyright 2014 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.
 
 import("//build/config/android/config.gni")
 import("//build/config/arm.gni")
 import("//build/config/ui.gni")
 import("//build/config/linux/pkg_config.gni")
 import("//media/media_options.gni")
 
@@ skipping 24 matching lines @@
 config("media_dependent_config") {
   if (is_win) {
     ldflags = [
       "/DELAYLOAD:mf.dll",
       "/DELAYLOAD:mfplat.dll",
       "/DELAYLOAD:mfreadwrite.dll",
     ]
   }
 }
 
+if (use_ozone) {
+  action("generate_ozone_constructor_list") {
+    # Ozone platform objects are auto-generated using similar
+    # patterns for naming and classes constructors. Here we build the
+    # object MediaOzonePlatform.
+    script = "../ui/ozone/generate_constructor_list.py"
+
+    platform_list_txt_file = "$target_gen_dir/ui/ozone/platform_list.txt"
+    constructor_list_cc_file = "$target_gen_dir/media/ozone/constructor_list.cc"
+
+    sources = [ platform_list_txt_file ]
+    outputs = [ constructor_list_cc_file ]
+    args = [
+      "--platform_list=$platform_list_txt_file",
+      "--output_cc=$constructor_list_cc_file",
+      "--namespace=media",
+      "--typename=MediaOzonePlatform",
+      "--include=\"media/ozone/media_ozone_platform.h\""
+    ]
+  }
+}
+
 component("media") {
   sources = [
     "base/audio_block_fifo.cc",
     "base/audio_block_fifo.h",
     "base/audio_buffer.cc",
     "base/audio_buffer.h",
     "base/audio_buffer_queue.cc",
     "base/audio_buffer_queue.h",
     "base/audio_capturer_source.h",
     "base/audio_buffer_converter.cc",
@@ skipping 444 matching lines @@
       ]
       sources += [
         "base/user_input_monitor_linux.cc"
       ]
     } else {
       defines += [ "DISABLE_USER_INPUT_MONITOR" ]
     }
   }
 
   if (use_ozone) {
-    platform_list_txt_file = "$target_gen_dir/ui/ozone/platform_list.txt"
-    constructor_list_cc_file = "$target_gen_dir/media/ozone/constructor_list.cc"
-
     # Used for the generated listing header (ui/ozone/platform_list.h)
     include_dirs += [ target_gen_dir ]
 
     sources += [
-      constructor_list_cc_file,
       "ozone/media_ozone_platform.cc",
       "ozone/media_ozone_platform.h",
+    ] + get_target_outputs(":generate_ozone_constructor_list")
+
+    deps += [
+      ":generate_ozone_constructor_list",
+      "//ui/ozone",
     ]
-
-    deps += [ "//ui/ozone/ozone" ]
-
-    action("generate_constructor_list") {
-      # Ozone platform objects are auto-generated using similar
-      # patterns for naming and classes constructors. Here we build the
-      # object MediaOzonePlatform.
-      script = "../ui/ozone/generate_constructor_list.py"
-      sources = [ platform_list_txt_file ]
-      outputs = [ constructor_list_cc_file ]
-      args += [
-        "--platform_list=$platform_list_txt_file",
-        "--output_cc=$constructor_list_cc_file",
-        "--namespace=media",
-        "--typename=MediaOzonePlatform",
-        "--include=\"media/ozone/media_ozone_platform.h\""
-      ]
-    }
   }
 
   if (is_mac) {
     libs += [
       "CoreMIDI.framework",
       "CoreVideo.framework",
       "OpenGL.framework",
       "QTKit.framework",
     ]
   }
@@ skipping 626 matching lines @@
       "base/android/java/src/org/chromium/media/ImageFormat.template",
     ]
     inputs = [
       "video/capture/android/imageformat_list.h"
     ]
     package_name = "org/chromium/media"
   }
 
   # TODO(dalecurtis): Finish media_unittests_apk and media_perftests_apk.
 }