Update ffmpeg.

DEPS

 vars = {
   # Use this googlecode_url variable only if there is an internal mirror for it.
   # If you do not know, use the full path while defining your new deps entry.
   "googlecode_url": "http://%s.googlecode.com/svn",
   "webkit_trunk": "http://svn.webkit.org/repository/webkit/trunk",
   "nacl_trunk": "http://src.chromium.org/native_client/trunk",
   "webkit_revision": "79731",
   "chromium_git": "http://git.chromium.org/git",
   "swig_revision": "69281",
   "nacl_revision": "4382",
   "libjingle_revision": "50",
   "libvpx_revision": "75967",
-  "ffmpeg_revision": "76055",
+  "ffmpeg_revision": "76139",
   "skia_revision": "820",
   "v8_revision": "6943",
 }
 
 deps = {
   "src/breakpad/src":
     (Var("googlecode_url") % "google-breakpad") + "/trunk/src@774",
 
   "src/build/util/support":
     "/trunk/deps/support@20411",
@@ skipping 300 matching lines @@
 ]
 
 
 hooks = [
   {
     # A change to a .gyp, .gypi, or to GYP itself should run the generator.
     "pattern": ".",
     "action": ["python", "src/build/gyp_chromium"],
   },
 ]