Check Attrribution-Reporting-Eligible header with structured parser

Rather than string comparisons. The exact serialization of this header,
which contains a structured dictionary, is not guaranteed, and may be
subject to greasing in the future.

Bug: 1446382
Change-Id: I7ec7e12ae0e736df736d720c2f6c4d21f7e7514b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4544663


Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Garrett Tanzer <gtanzer@chromium.org>
Commit-Queue: Andrew Paseltiner <apaseltiner@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1146534}