Reference existing ffmpeg GN buildfile from GN build.

This file is already landed in the ffmpeg repo and rolled into Chrome. This change just references it so that it will build.

TBR=ajwong@chromium.org

Review URL: https://codereview.chromium.org/342033002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@278444 0039d316-1c4b-4281-b951-d872f2087c98

BUILD.gn

@@ -62,6 +62,7 @@ group("root") {
     "//sql",
     "//third_party/angle:translator",
     "//third_party/brotli",
+    "//third_party/ffmpeg",
     "//third_party/flac",
     "//third_party/harfbuzz-ng",
     "//third_party/iccjpeg",
@@ -161,6 +162,7 @@ group("root") {
       "//third_party/WebKit/public:all_blink",
 
       # Not tested on Android yet:
+      "//third_party/ffmpeg",
       "//webkit/browser:storage",
       "//webkit/child",
     ]