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Enables HEADLESS operation on screen-less devices, using a switch
published via the system's config store.

Reading/parsing logic is promoted to a common library,
"config_reader.h", and ContextProvider is modified to use it.

Bug: 1025045
Change-Id: Iad72297cf0e2a2d30e2650563c23d58204ad7eb5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1989998
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#729920}