Check timestamp of .ninja_log before upload and don't upload if mtime is very old.

Bug: 900161
Change-Id: I0cf00ae29e23a9f609564e4c12b4b518320ca23a
Reviewed-on: https://chromium-review.googlesource.com/c/1404611
Commit-Queue: Shinya Kawanaka <shinyak@chromium.org>
Reviewed-by: Shinya Kawanaka <shinyak@chromium.org>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>