Explicitly use gclient.bat in gclient_sync_retry on Windows

b/35473293

Change-Id: Icf135750fd9c60253ad677790c7d9efaa92298aa
diff --git a/gclient_sync_retry.py b/gclient_sync_retry.py
index 7fd17c0..7588167 100644
--- a/gclient_sync_retry.py
+++ b/gclient_sync_retry.py
@@ -13,6 +13,7 @@
 # limitations under the License.
 """Gclient sync, retry if failed to connect."""
 
+import os
 import random
 import subprocess
 import sys
@@ -21,7 +22,7 @@
 
 def main():
   command = [
-      'gclient',
+      ('gclient' if os.name != 'nt' else 'gclient.bat'),
       'sync',
       '--verbose',
       # Calls git reset --hard HEAD on each