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..dfa9801a6a35c68dd0fa0f100fa1ff50ed111f8d 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -1069,6 +1069,26 @@ 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.
+# 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"