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

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index d595dc8e6f4afdd1c1f78edb21a9c50c04e9c158..098db0072d50d99f09ab8c13360313b8cb6d6901 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -1069,6 +1069,15 @@ config("no_size_t_to_int_warning") {
   }
 }
 
+# Some code presumes that pointers to structures/objects are compatible
+# regardless of whether what they point to is already known to be valid.
+config("no_incompatible_pointer_warnings") {
+  cflags = []
+  if (is_clang) {
+    cflags += [ "-Wno-incompatible-pointer-types" ]

[jamesr, 2015/10/20 23:06:54]

we should open a bug on this, this is not a good warning to disable (i.e. the
ffmpeg code depending on this is wrong)

+  }
+}
+
 # Optimization -----------------------------------------------------------------
 #
 # Note that BUILDCONFIG.gn sets up a variable "default_optimization_config"