Rewrite ffmpeg_binaries target to use GYP built-ins and branding conditions.

third_party/ffmpeg/copy_binaries.sh

-#!/bin/bash
-#
-# This is meant to replicate the behavior of cp, except that it will not fail
-# if the source files are not present.  Something like
-# "cp ${SOURCES} ${DEST} || true" would also have worked except that
-# gyp does not allow for specifying || in the action, and the windows
-# cygwin envrionment does not include "true" making a script like this the
-# least ugly solution.
-
-SOURCES=""
-DESTINATION=""
-
-# Shift off every argument but the last and consider them the sources.
-# It would have probably been easier to put the destination first, but
-# this is not too hard and it replicates the argument ordering of cp.
-while (( "$#" != 1 )); do
-  SOURCES="$SOURCES $1"
-  shift
-done
-
-DESTINATION=$1
-
-# Early out if there was not enough parameters to discern a destination.
-# Also fail the command because this means we are being invoked incorrectly.
-if test -z "$DESTINATION"; then
-  echo "ERROR: Destination empty."
-  exit 1
-fi
-
-# Only try to copy the source file if it exists.  It is not an error
-# if the input does not exist; we just silently ignore the input.
-for i in $SOURCES; do
-  if test -f $i; then
-    cp -v -f $i $DESTINATION
-  fi
-done
-
-# Make sure we always succeed.
-exit 0