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Reland: Add more unit tests for url_lookup_service.

Add tests for the StartLookup method.

The original CL was reverted https://crrev.com/c/2089440.
The error is "Trying to register a previously registered pref"
Remove the double registered pref in this CL and reland.

Bug: 1050859
Change-Id: Ia5f88145f6c7fdbe98b7c1adf8e020602f341437
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2091593Reviewed-by: Daniel Rubery <drubery@chromium.org>
Commit-Queue: Xinghui Lu <xinghuilu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747885}