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This CL changes how search engines are displayed in Search Engine Settings.

Previous(1 list):
  1. search engines that are prepopulated or created by policy.

Current(2 lists):
  1. Search engines that are prepopulated or created by policy, and possibly
     one custom search engine at the end if it's selected as default search
     engine;
  2. All remaining search engines.

Unittests are also updated.

Bug: 433824
Change-Id: Ib70a18f0544101760c331bb349ee9818956ed231
Reviewed-on: https://chromium-review.googlesource.com/c/1323509Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Yi Su <mrsuyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606776}