IWYU net/ again

even after
  https://webrtc-review.googlesource.com/c/src/+/392040
IWYU found issues in net/.

Another pass of
  find net -name "*.h" -o -name "*.cc" | xargs tools_webrtc/iwyu/apply-include-cleaner
followed by
  find net -name "*.h" -o -name "*.cc" | xargs sed -i '/<stdint.h>/d'
  find net -name "*.h" -o -name "*.cc" | xargs sed -i '/<stddef.h>/d'
followed by another pass of
  find net -name "*.h" -o -name "*.cc" | xargs tools_webrtc/iwyu/apply-include-cleaner
then
  tools_webrtc/gn_check_autofix.py -C out/Default/
and
  git cl format

Manual changes:
tlv_trait.h: replaced string.h with cstring
chunk_validators.cc: added "absl/algorithm/container.h"

Bug: webrtc:42226242
Change-Id: I7b562e12ed9f6f4b0a21a6095eee70b2bea0b9b9
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/396360
Commit-Queue: Philipp Hancke <phancke@meta.com>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Evan Shrubsole <eshr@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#45013}