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This adds support for the generalized 'List' syntax in
https://tools.ietf.org/html/draft-ietf-httpbis-header-structure-13.
Since some types have changed slightly between revisions, the parser now
takes a version parameter on construction, which can be used to switch
between draft 9 (for compatibility with existing Web Packaging
deployments) and draft 13 (for new headers).

Bug: 1011101
Change-Id: Ie8a76384430f5319e1d564e51f4f62af7c0ada7f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872764
Commit-Queue: Ian Clelland <iclelland@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713847}