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Adds a experiment param to require the user to have Lite Mode enabled,
default true for the time being.

Also removes the integration with the Data Saver config to get the proxy
tunnel hosts and chrome-proxy header.
Now, the tunnel host will be given through experiment param or cmd line
and uses a tunnel header with the Google API key.

Also fixes some flakiness in the new probing tests.

Bug: 1135213
Change-Id: I23802a5c9787abe669759be0f782b3d586afd7dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2449912
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815239}