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- Record settings histogram also when unified consent is disabled.

- Add browser test class for the UnifiedConsentService and add
  tests for the settings histogram.
  Note: More tests for the consent service will be added in a
  follow-up CL. Test support for Android and ChromeOS will be
  added in the future.

Bug: 908061, 888527
Change-Id: I4fa3ade5b8d3147c40032a3da631523e94ca09a3
Reviewed-on: https://chromium-review.googlesource.com/c/1352255
Commit-Queue: Thomas Tangl <tangltom@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612188}