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This adds an about:flags entry for real-time Safe Browsing
lookups, to make it easier to enable and test.

Bug: 1103213
Change-Id: I28bae0f333ece6d1f5f8f4dcf5b3c89ef6037772
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315236
Commit-Queue: Ali Juma <ajuma@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791295}