Add global 300s global timeout for Gerrit API calls

This will not impact the calls which have explicit timeout.
For the rest, either PUT or POST or DELETE methods, 5 min should
suffice.

Fixed: 1441863
TEST=NA

Change-Id: I235bc60344b6981eb3d96930ca8b6463129a2be2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4495994
Commit-Queue: Xinan Lin <linxinan@chromium.org>
Reviewed-by: Gavin Mak <gavinmak@google.com>

gerrit_util.py

@@ -380,7 +380,7 @@ def CreateHttpConn(host,
                    reqtype='GET',
                    headers=None,
                    body=None,
-                   timeout=None):
+                   timeout=300):
   """Opens an HTTPS connection to a Gerrit service, and sends a request."""
   headers = headers or {}
   bare_host = host.partition(':')[0]