chromium: Install libffmpegsumo.so.

This caused HTML5 video to not work because this shared library is loaded at
runtime.

Unfortunately we can't use system ffmpeg yet, because upgrading would break
builds of other packages, and it would result in a copy of ffmpeg laying around
aswell, so we can defer this until we have fixed ffmpeg.

Thanks to @bluescreen303 for the bug report.

pkgs/applications/networking/browsers/chromium/default.nix

@@ -172,6 +172,7 @@ in stdenv.mkDerivation rec {
     mkdir -vp "$out/libexec/${packageName}"
     cp -v "out/${buildType}/"*.pak "$out/libexec/${packageName}/"
     cp -vR "out/${buildType}/locales" "out/${buildType}/resources" "$out/libexec/${packageName}/"
+    cp -v out/${buildType}/libffmpegsumo.so "$out/libexec/${packageName}/"
 
     cp -v "out/${buildType}/chrome" "$out/libexec/${packageName}/${packageName}"