Run clang-tidy modernize-use-nullptr on //net

An instance of GetAddressOf() in url_security_manager_win.cc has also been updated to allow presubmit checks to pass.

This is a mechanical change.
See bugs for justification and details.

Bug: 776257, 778942
Change-Id: Ia8bb79e611be7a884227852224623043af8b850e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1512637
Reviewed-by: Bence Béky <bnc@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Raul Tambre <raul@tambre.ee>
Cr-Commit-Position: refs/heads/master@{#639546}
262 files changed