refactor: do not use electron.gyp contents in the verify-ffmpeg script

Also run verify ffmpeg with cmd instead of powershell

script/lib/gn.py

@@ -1,6 +1,7 @@
 #!/usr/bin/env python
 
 import subprocess
+import sys
 
 from util import scoped_cwd
 
@@ -9,9 +10,15 @@ class GNProject:
   def __init__(self, out_dir):
     self.out_dir = out_dir
 
+  def _get_executable_name(self):
+    if sys.platform == 'win32':
+      return 'gn.bat'
+
+    return 'gn'
+
   def run(self, command_name, command_args):
     with scoped_cwd(self.out_dir):
-      complete_args = ['gn', command_name, '.'] + command_args
+      complete_args = [self._get_executable_name(), command_name, '.'] + command_args
       return subprocess.check_output(complete_args)
 
   def args(self):