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- Add a equivalent_result_id field to search result;
- OmniboxResult uses stripped search url as its id;
- AnswerCardResult sets equivalent_result_id to a stripped search
  url that is used by search-what-user-typed omnibox result;
- SearchAnswerCardView use the equivalent_result_id to delete
  relevant result when the card is loaded;
- Remove the unused comparable_id from ChromeSearchResult;

Bug: 906890
Test: SearchResultAnswerCardViewTest.RemoveEquivalent
Change-Id: I16529724c8a86790ed1b5366095e2701118f9231
Reviewed-on: https://chromium-review.googlesource.com/c/1351714Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Weidong Guo <weidongg@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611390}