[iOS] Fix Async Safe Browsing Test Logic

Having a method to pause sync or async checks isn't needed anymore now
that we can control this by calling the function that runs an array of
callbacks. Additionally, the async logic branch for pending main frame
queries didn't take into account logic for cancelling navigation. This
should be added so that logic isn't waiting for sync queries to run
cancel the navigation when a decision is already found from the case
where an async check happened before a sync check happened.

The current logic in FakeSafeBrowsingService is incorrect since async
checks aren't stored unless async responses are paused and in the new
test infrastructure, by default, both sync and async checks are
"paused" until we run the callbacks at a later point.

Bug: 337243708
Change-Id: I79736f001d177f39a5bb9a948d819c65f9594900
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5866249
Reviewed-by: Ali Juma <ajuma@chromium.org>
Auto-Submit: Joemer Ramos <joemerramos@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Ali Juma <ajuma@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1356194}