WhitelistURLSet keeps a count of how many times a URL has been added to
the pending whitelist, and when removing, it decreases the count by one
, onlyremoving a URL if the count reaches zero. This solves a problem
where upon reloading a safe browsing interstitial, the website would be
re-added and removed from the whitelist in the wrong order, causing it
to not be in the whitelist when it was supposed to.

Bug: 666172
Change-Id: Ic870153381685890b5c3bc99ad2133837ffe34d7
Reviewed-on: https://chromium-review.googlesource.com/671672


Commit-Queue: Carlos IL <carlosil@chromium.org>
Reviewed-by: Jialiu Lin <jialiul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#503335}