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When making query to swarming for bots with dimension values, it is
possible that no bot is returned. Normally, the return JSON looks
like this:
{
  "death_timeout": "xxx",
  "items": ["list","of","bots"]
  "now": "xxx"
}
When there's no bot, the "items" key is gone:
{
  "death_timeout": "xxx",
  "now": "xxx"
}
In this case, we will see a KeyError in pre_run stage because we try to
read values using "items" key in perf_device_trigger.py:
  for bot in query_result['items']:
    ...

KeyError doesn't tell what is actually happening. To fix, we check this
key before using it. Then when no bot exists, we will see a message saying
there's not enough machines.


Bug: chromium:1027147
Change-Id: I38fe0e1a015e29d9b223515f49c527641bcebf60
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1929871Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Commit-Queue: Caleb Rouleau <crouleau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718356}