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This CL makes always ThumbnailDatabase::RetainDataForPageUrls()  drop
Page URL -> Icon URL mappings for unretained page URLs.

Previously, a Page URL -> Icon URL (e.g. PageURL1 -> IconURLA) mapping was not
dropped for an unretained Page URL (PageURL1) if:
- the favicon data for a different page URL (e.g. PageURL2) was retained
AND
- PageURL2 is mapped to the same Icon URL as the unretained page URL
  (PageURL2 -> IconURLA)

BUG=468353
TEST=ThumbnailDatabaseTest.RetainDataForPageUrls

Review URL: https://codereview.chromium.org/1005783003

Cr-Commit-Position: refs/heads/master@{#321221}