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If the fetch of the warmup URL (aka probe URL) fails, the
corresponding proxy is disabled, and the config is reloaded.

A future CL will add retries -- If the fetch of the probe URL
due to network flakiness fails, then the probe will be retried
(up to 3 times) on the same proxy.

Bug: 760294
Change-Id: Ib255f8178ef106f5e2882ad3dc32bde143c40a90
Reviewed-on: https://chromium-review.googlesource.com/807592Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Doug Arnett <dougarnett@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521951}