This is full version of
https://chromium-review.googlesource.com/823544

After the CL landing, we see no performance regression in bot_update step.
And regular 95%ile in 1h window improved from more than 300s to less than or around 200s.
See tandrii's query in https://bugs.chromium.org/p/chromium/issues/detail?id=749709#c91 for detail.

Let me reduce frequency of slow bot_update step more like below.
https://ci.chromium.org/buildbot/tryserver.chromium.win/win7_chromium_rel_ng/69844
https://ci.chromium.org/buildbot/tryserver.chromium.win/win7_chromium_rel_ng/69886
https://ci.chromium.org/buildbot/tryserver.chromium.win/win7_chromium_rel_ng/69892

Bug: 749709
Change-Id: I44474aa9ecdd733d68fd13fe8b791820fb80381f
Reviewed-on: https://chromium-review.googlesource.com/838766
Commit-Queue: Takuto Ikuta <tikuta@google.com>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>