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Fixes use-after-free issues in URLChecker and
KidsManagementURLCheckerClient.

Asynchronous callbacks passed to unowned objects should be bound with
weak pointers, so if `this` is destroyed, the callback won't fire.

Test:
* As a parent, set a child's Safe Site setting to "Allow all sites"
* Add the child to a Chromebook
This CL fixes DCHECKs and (in prod) potential use-after-free segfaults
that could otherwise occur when running the steps above.

Bug: 1159898
Change-Id: I67ba8f924ded3b6316813529e9b77a3d3735b483
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2597547Reviewed-by: Aga Wronska <agawronska@chromium.org>
Reviewed-by: Toby Huang <tobyhuang@chromium.org>
Commit-Queue: Aga Wronska <agawronska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843171}