GitLab

Use the post method from requests.Session so connection can be reused,
saving reporting time and potentially fix a connection error.

Added a finalize method to StdJson class, to close the connection after
reporting all results.

Bug: 1136998
Change-Id: I7e5c5e40b700655f4ac32ca8f84813c9b35cce5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521514Reviewed-by: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Commit-Queue: Zhaoyang Li <zhaoyangli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826653}