swarming: ensure the external scheduler's slice is active
swarming: ensure the external scheduler's slice is active

BUG=chromium:912776
TEST=Local tests with dev instance

Change-Id: I2e3690bf0670bfad9e1e30914ff8895982a95970
Reviewed-on: https://chromium-review.googlesource.com/c/1372351
Reviewed-by: Aviv Keshet <178e8ae417efbe404feca7ab4cb8f0ba8c109369@chromium.org>
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py