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

build/config/BUILDCONFIG.gn

Index: build/config/BUILDCONFIG.gn
diff --git a/build/config/BUILDCONFIG.gn b/build/config/BUILDCONFIG.gn
index 16495470cae54d88b9d9edb535819431627d00dc..31e187efe7ee2ae476ab72c5169975d816c5f9e7 100644
--- a/build/config/BUILDCONFIG.gn
+++ b/build/config/BUILDCONFIG.gn
@@ -426,6 +426,9 @@ if (is_debug) {
 }
 _native_compiler_configs += [ _default_optimization_config ]
 
+# //third_party/ffmpeg/BUILD.gn wants to remove this, add it. Does nothing.

[brettw, 2015/10/13 21:35:36]

You should migrate to Chrome's configuration for this, where the default
optimization config is added unconditionally and this expands to the current
default behavior. We do the same for symbols, etc now since it makes it much
easier to do what ffmpeg is doing. If you don't migrate to this, you'll lose
optimization for the targets that opt into max-optimization.

+_native_compiler_configs += [ "//build/config/compiler:default_optimization" ]
+
 # If it wasn't manually set, set to an appropriate default.
 if (symbol_level == -1) {
   # Linux is slowed by having symbols as part of the target binary, whereas