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In //weblayer, SafeBrowsingDBManager currently can can be accessed only
from the IO thread. However, the subresource filter component needs to
take in a scoped_refptr to the embedder's SafeBrowsingDBManager on the
UI thread as part of creating a navigation throttle. This instance is
then internally passed to and accessed on the IO thread.

In //chrome, the ServicesDelegate class creates the
SafeBrowsingDBManager on the UI thread and vends a reference to it
on any thread. This CL takes an analogous approach in //weblayer,
allowing the method that creates the SafeBrowsingDBManager to be
called on the UI thread, with the DBManager being *started* on the IO
thread as it is now (via a posted task if the current thread is not the
IO thread). We add an IO-thread safeguard to  ensure that the DBManager
is only started once.

The precise structure here is not the same as that in //chrome's
SafeBrowsingService, as the underlying structure is not the same:
//chrome's SafeBrowsingService has an explicit Start() method from
which it posts a task to start the SafeBrowsingDBManager on the IO
thread. //weblayer's SafeBrowsingService has no such method and
instead the SafeBrowsingDBManager is created/started on demand.

Bug: 1116095
Change-Id: If6303ee0ec36ec75109eb3917dd5cea3946d98bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2478950
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Tim Volodine <timvolodine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818850}