[iOS] Re-enable Safe Browsing full-hash requests

This reverts commit 49a8b1ff42853c34e74d158350ccb75017a98add.

Reason for revert: M83 has branched, and we want full-hash requests
to be enabled in M84.

Original change's description:
> Reland "[iOS] Disable Safe Browsing full-hash requests"
>
> This is a reland of 6a3e1c15a84e55c49358d155af3412a3d9b4ca97.
> Relanding since M83 is branching soon. This will be reverted
> again after the branch point.
>
> Original change's description:
> > [iOS] Disable Safe Browsing full-hash requests
> >
> > This temporarily disables making full-hash requests on iOS, allowing
> > the rest of the Safe Browsing code to be exercised.
> >
> > Since this change breaks unit tests, this CL also stops running
> > Safe Browsing database unit tests on iOS for now.
> >
> > Bug: 1028755
> > Change-Id: Id772d4682bd9a3443b8d6b7042b0b5aecaaff80e
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2088349
> > Reviewed-by: Varun Khaneja <vakh@chromium.org>
> > Commit-Queue: Ali Juma <ajuma@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#748049}
>
> Bug: 1028755
> Change-Id: Ib63d6020eaeb3d53b617f2f903f2130266993fe8
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2122457
> Reviewed-by: Varun Khaneja <vakh@chromium.org>
> Commit-Queue: Ali Juma <ajuma@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#753728}

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 1028755
Change-Id: I769b5b6b464743cdd7efbd64f749cd359d6939d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2161788
Commit-Queue: Ali Juma <ajuma@chromium.org>
Commit-Queue: Varun Khaneja <vakh@chromium.org>
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#761691}