Fix //media:media_unittests for gn

BUG=

Review URL: https://codereview.chromium.org/972473005

Cr-Commit-Position: refs/heads/master@{#319198}

BUILD.gn

@@ -304,7 +304,7 @@ if (is_linux) {
       "//ipc/mojo:ipc_mojo_unittests",  # PASSES 2/25/2015
       "//jingle:jingle_unittests",  # PASSES 2/25/2015
       "//media/cast:cast_unittests",  # PASSES 2/25/2015
-      "//media:media_unittests",  # TODO(GYP) MidiManagerTest fails.
+      "//media:media_unittests",  # PASSES 3/3/2015
       "//mojo/common:mojo_common_unittests",  # PASSES 2/25/2015
       "//net:net_unittests",  # PASSES 2/25/2015
       "//ppapi:ppapi_unittests",  # PASSES 2/26/2015

media/BUILD.gn

@@ -32,6 +32,9 @@ config("media_config") {
   if (use_cras) {
     defines += [ "USE_CRAS" ]
   }
+  if (use_alsa) {
+    defines += [ "USE_ALSA" ]
+  }
 }
 
 config("media_dependent_config") {
@@ -352,7 +355,6 @@ component("media") {
 
   if (use_alsa) {
     libs += [ "asound" ]
-    defines += [ "USE_ALSA" ]
     sources += [
       "midi/midi_manager_alsa.cc",
       "midi/midi_manager_alsa.h",