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Step 1 of 3 for caching prober results. This CL adds caching by network
id (more or less just the connection type), and the URL host:port being
probed.

Future CLs on this thread:
* Step 2 - async revalidate cache entries after some expiry
* Step 3 - persist to disk with PrefService

Bug: 977603
Change-Id: If84f74e4cdf29eba16f864c34e08170d65ee2d20
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1691548
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#675794}