Reference existing ffmpeg GN buildfile from GN build.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index f7fdde9c05bb4c4926714ef83153274eeef5784d..d0133a46e451bd80825e0dc9958c7b6e93a42aca 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -58,6 +58,7 @@ group("root") {
     "//sql",
     "//third_party/angle:translator",
     "//third_party/brotli",
+    "//third_party/ffmpeg",
     "//third_party/flac",
     "//third_party/harfbuzz-ng",
     "//third_party/iccjpeg",
@@ -156,6 +157,7 @@ group("root") {
       "//third_party/WebKit/Source/wtf",
 
       # Not tested on Android yet:
+      "//third_party/ffmpeg",
       "//webkit/browser:storage",
       "//webkit/child",
     ]