Also redirect stderr from the browser since Chrome recently started using it too

Example stderr output from Chrome:

  [24599:24650:0306/123817.372418:ERROR:browser_process_sub_thread.cc(209)]
  Waited 3 ms for network service

R=nodir@chromium.org

Change-Id: Idb71c9f3a2c735ee49b886896f55bdede559137c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1505792Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Commit-Queue: Sergiy Belozorov <sergiyb@chromium.org>

git_cl.py

@@ -5066,12 +5066,15 @@ def CMDweb(parser, args):
   # allows to hide "Created new window in existing browser session." message
   # from Chrome. Based on https://stackoverflow.com/a/2323563.
   saved_stdout = os.dup(1)
+  saved_stderr = os.dup(2)
   os.close(1)
+  os.close(2)
   os.open(os.devnull, os.O_RDWR)
   try:
     webbrowser.open(issue_url)
   finally:
     os.dup2(saved_stdout, 1)
+    os.dup2(saved_stderr, 2)
   return 0