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In this patch:
 - Add SafeBrowsingService and its implementation for managing
   initialization and ownership of safebrowsing related objects,
   such as database manager, ui manager and safebrowsing context.
   This also includes proper threading and startup of the database
   manager.
 - Move some functionality and ownership from ContentBrowserClient
   to SafeBrowsingService for better decoupling and less code
   complexity.
 - Add user agent and user data path for safebrowsing network
   context.
 - Make safebrowsing android only for now.
 - Update build files.

BUG=1015809

Change-Id: I8f83c62538bafe81479cb9fe40f065022e1699d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1888033Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Tim Volodine <timvolodine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711288}