Implement content negotiation using Variants and Variant-Key

This CL introduces SignedExchangeRequestMatcher::
FindBestMatchingVariantKey() method. This method is called from
AlternateSignedExchangeResourceInfo::FindMatchingEntry() by PreloadHelper and
PrefetchedSignedExchangeManager to decide which alternate subresource signed
exchange should be used.

Bug: 935267
Change-Id: I89520c27afc7efe8f2000f2c8acbecacff512a88
Commit-Queue: Tsuyoshi Horo <>
Reviewed-by: Kunihiko Sakamoto <>
Reviewed-by: Kinuko Yasuda <>
Reviewed-by: Kentaro Hara <>
Cr-Commit-Position: refs/heads/master@{#676287}
10 files changed