Fix crash in WebFaviconDriver::OnFaviconDeleted.

Followup to https://chromium-review.googlesource.com/758863
that forgot to fix one use of web_state(). This fix a crash
in WebFaviconDriver::OnFaviconDeleted.

Bug: 775684
Change-Id: I08910c647de2595606644136b5ab080e0e9e7300
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Reviewed-on: https://chromium-review.googlesource.com/771651
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516674}