PM: Work around UAF on destruction notification.

Avoid dispatching on a deleted SiteDataCacheImpl by using a WeakPtr
for the delegate.

This CL is a quick workaround for the larger problem, which is that the
WebUI's SiteDataProviderImpl needs a teardown notification when the
SiteDataCacheImpl is unregistered and/or deleted. This would allow the
data provider to clean up before deletion of the data provider, which
could then in turn assert that no site datums are held on its destruction.

Bug: 1231933
Change-Id: Ia50464f0f3dee6ded38f1b1b84f00f0ebd79fc9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3057116


Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: Sébastien Marchand <sebmarchand@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#906345}