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

Bug: 1028755
Change-Id: I48bb2c6d20a707a2f44810e71403abd2eb8dda3d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2105797
Commit-Queue: Ali Juma <ajuma@chromium.org>
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751691}