[iOS] Fix unit tests that fail when Safe Browsing is enabled

This CL makes FakeSafeBrowsingService::ClearCookies call
the callback it is given, fixing BrowsingDataRemoverImplTests
when Safe Browsing is enabled by default.

Bug: 1028755
Change-Id: Id586113c49a62133de4568b3dce35b9cdf17cae3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2438733
Commit-Queue: Ali Juma <ajuma@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Auto-Submit: Ali Juma <ajuma@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811862}

ios/chrome/browser/safe_browsing/fake_safe_browsing_service.mm

@@ -90,4 +90,5 @@ void FakeSafeBrowsingService::ClearCookies(
     const net::CookieDeletionInfo::TimeRange& creation_range,
     base::OnceClosure callback) {
   DCHECK_CURRENTLY_ON(web::WebThread::UI);
+  std::move(callback).Run();
 }