Description:
Once all C string style string processing is removed from HeaderString and all functionality has been migrated to absl::string_view, it should be safe to reclaim the NUL byte that currently needs to be stored at the end of the strings. This should be done separately as an independent PR once all blockers are resolved as it is a risky change.

Context:
#6580 must be resolved first.

Action item for CVE-2019-9900