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This changes SafeBrowsingService into a pure virtual interface, moves the
implementation to a new SafeBrowsingServiceImpl, and creates a new
FakeSafeBrowsingService implementation. This fake implementation creates
FakeSafeBrowsingUrlCheckerImpls that treat all queried URLs as safe.

This CL uses FakeSafeBrowsingService in TestingApplicationContext, which
lets us simplify SafeBrowsingTabHelper, since it no longer needs to handle
the case where the current ApplicationContext returns a null
SafeBrowsingService.

Change-Id: I515e0e53a28b4f9173fcae892dd993e159dbc7b5
Bug: 1060300
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2106804
Commit-Queue: Ali Juma <ajuma@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762532}