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This creates an iOS version of SafeBrowsingService, which handles creating
and initializing a SafeBrowsingDatabaseManager, and also handles starting
and stopping the database manager depending on the value of the
Safe Browsing opt-out preference.

Behind the kSafeBrowsingAvailableOnIOS flag, this creates a
SafeBrowsingServiceIOS owned by ApplicationContextImpl and initialized during
IOSChromeMainParts::PreMainMessageLoopRun.

Change-Id: I5eaf3ee1a92ab55ef84eb38936a4818a246f6df9
Bug: 1028755,1049327
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2044521
Commit-Queue: Ali Juma <ajuma@chromium.org>
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Reviewed-by: Maksim Orlovich <morlovich@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748268}