Add profile-awareness to realtime URL lookups

The BrowserURLLoaderThrottle now checks the current BrowserContext to
determine whether real-time checks are enabled at all, then passes that
information to the SafeBrowsingUrlCheckerImpl.

Bug: 991394, 963165, 1001566
Change-Id: I5dbff848a4ea5a6ae3ac5c76e5d5624596ee3616
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1825482
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702956}