Factorize media/audio into new GN.

Breaks out one of the most complex media/ targets into its
own BUILD.gn file for improved readibility and targetting.

BUG=none
TEST=media_unittests

Review URL: https://codereview.chromium.org/396793003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284314 0039d316-1c4b-4281-b951-d872f2087c98

DEPS

@@ -220,7 +220,7 @@ deps = {
 
   "src/third_party/ffmpeg":
     Var("chromium_git") +
-    "/chromium/third_party/ffmpeg.git@4b7b36f956209fad838ad87d0129130713fbb00c",
+    "/chromium/third_party/ffmpeg.git@35b6086d85dd8aa7d29c2525e11debc5937bb506",
 
   "src/third_party/libjingle/source/talk":
     (Var("googlecode_url") % "webrtc") + "/trunk/talk@" +