Fix crash during shutdown caused by content verification code
Fix crash during shutdown caused by content verification code

Inside ContentVerifier::CreateJobFor(), we were accessing the
ExtensionRegistry even though shutdown may be in progress and it is
being torn down. The quick fix here is to avoid doing that by
early-returning if the fetcher or delegate is null (the entire feature
is turned off by default and the fetcher object is only created when the
feature is turned on, so this should stop the crash).

Looking closely at this code made me realize we are also accidentally
accessing the ExtensionRegistry from a non-UI thread, so in a follow-up
patch I will fix that as well.

BUG=389217

Review URL: https://codereview.chromium.org/358693008

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@280709 0039d316-1c4b-4281-b951-d872f2087c98

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