[mapper] Don't divide by float 0 when calculating shard ETA.
[mapper] Don't divide by float 0 when calculating shard ETA.

For some reason this gives ETA somewhere in 18th century, around the time
Peter I reformed the Russian Orthodox Church.

R=40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org

Change-Id: I89cc21c721665bef8f87c08164faa7286d1a02ff
Reviewed-on: https://chromium-review.googlesource.com/c/1273926
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go