Fix component build error in GN build.

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 630 matching lines @@
     "base/audio_bus.cc",
     "base/audio_bus.h",
     "base/channel_layout.cc",
     "base/channel_layout.h",
     "base/limits.h",
     "base/media_export.h",
     "base/vector_math.cc",
     "base/vector_math.h",
   ]
   configs += [ ":media_config" ]
+  defines = [ "MEDIA_IMPLEMENTATION" ]
   deps = [ "//base" ]
 }
 
 if (media_use_ffmpeg) {
   test("ffmpeg_unittests") {
     sources = [ "ffmpeg/ffmpeg_unittest.cc" ]
 
     deps = [
        ":media",
        ":test_support",
@@ skipping 59 matching lines @@
     deps = [
       ":media",
       ":shared_memory_support",
       "//base",
       "//ui/gl",
       "//ui/gfx",
       "//ui/gfx/geometry",
     ]
   }
 }