[swarming] clean up directory before downloading content
[swarming] clean up directory before downloading content

This is to prevent failure happening when retrying GetTaskOutputs.

Bug: 928952
Change-Id: I321b285e80ed6098b01ef453d2fca78a7042a516
Reviewed-on: https://chromium-review.googlesource.com/c/1454002
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

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