Add ffmpeg (chromium's branch) to third_party for use in mojo media.

build/config/compiler/BUILD.gn

 # Copyright (c) 2013 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/chrome_build.gni")
 if (current_cpu == "arm") {
   import("//build/config/arm.gni")
 }
 if (current_cpu == "mipsel" || current_cpu == "mips64el") {
@@ skipping 1051 matching lines @@
 # Note that this can be applied regardless of platform and architecture to
 # clean up the call sites. This will only apply the flag when necessary.
 #
 # TODO(jschuh): crbug.com/167187 fix this and delete this config.
 config("no_size_t_to_int_warning") {
   if (is_win && current_cpu == "x64") {
     cflags = [ "/wd4267" ]
   }
 }
 
+# Some code presumes that pointers to structures/objects are compatible
+# regardless of whether what they point to is already known to be valid.
+# gcc 4.9 and earlier had no way of suppressing this warning without
+# supressing the rest of them.  Here we centralize the identification of
+# the gcc 4.9 toolchains.

[jamesr, 2015/10/13 00:13:56]

I'm pretty sure we will never use a gcc 4.9 toolchain so I don't think we need
the thing this comment is referring to

+config("no_incompatible_pointer_warnings") {
+  cflags = []
+  if (is_clang) {
+    cflags += [ "-Wno-incompatible-pointer-types" ]
+  } else if (current_cpu == "mipsel") {

[jamesr, 2015/10/13 00:13:56]

we aren't building anything targeting mips

+    cflags += [ "-w" ]
+  } else if (is_chromeos && current_cpu == "arm") {

[jamesr, 2015/10/13 00:13:56]

we aren't building for chromeos

+    cflags += [ "-w" ]
+  }

[jamesr, 2015/10/13 00:13:56]

was there supposed to be something gcc 4.9 related here?

+}
+
+# //third_party/ffmpeg/BUILD.gn wants to remove this, so we need a placeholder.
+config("default_optimization") {
+}
+
 # Optimization -----------------------------------------------------------------
 #
 # Note that BUILDCONFIG.gn sets up a variable "default_optimization_config"
 # which it will assign to the config it implicitly applies to every target. If
 # you want to override the optimization level for your target, remove this
 # config (which will expand differently for debug or release builds), and then
 # add back the one you want to override it with:
 #
 #   configs -= default_optimization_config
 #   configs += [ "//build/config/compiler/optimize_max" ]
@@ skipping 164 matching lines @@
       cflags += [ "-gsplit-dwarf" ]
     }
   }
 }
 
 config("no_symbols") {
   if (!is_win) {
     cflags = [ "-g0" ]
   }
 }