GN: Re-enable platform unit test targets

On linux use_webaudio_ffmpeg is true and when it is platform needs to
depend on //third_party/ffmpeg. This target is set up in the GN build now
(at least for this platform), so it's safe to re-enable.

R=brettw@chromium.org

Review URL: https://codereview.chromium.org/353943003

git-svn-id: svn://svn.chromium.org/blink/trunk@177142 bbb929c8-8fbe-4397-9dbb-9b2b20218538