[swarming] Wait indefinitely if timeout is 0
[swarming] Wait indefinitely if timeout is 0

This change fixes the swarming client to actually wait indefinitely, as
the usage suggests, when timeout is unspecified, or set to zero.

Change-Id: I7e4d7ba0343c2793afa37ec6ded8ebe8f86ae533
Reviewed-on: https://chromium-review.googlesource.com/935299
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Commit-Queue: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>

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