Introduce CompressionDictionaryTransportRequireKnownRootCert flag

We are observing potential issues with CompressionDictionaryTransport
feature that may be caused by network appliances that intercept network
traffic.

To investigate this issue, this CL introduces a new feature flag named
CompressionDictionaryTransportRequireKnownRootCert. If this feature is
enabled, Chromium will use stored shared dictionaries only when the
HTTPS connection's certificate is rooted by a well known root CA or when
the server is localhost.

Bug: 1413922
Change-Id: Iaaa9d4db9d335cde12632925c1e36b2a55a4cf9d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5028336
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Christian Dullweber <dullweber@chromium.org>
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1232707}

• chrome/browser/about_flags.cc[diff]
• chrome/browser/browsing_data/browsing_data_model_browsertest.cc[diff]
• chrome/browser/flag-metadata.json[diff]
• chrome/browser/flag_descriptions.cc[diff]
• chrome/browser/flag_descriptions.h[diff]
• docs/experiments/compression-dictionary-transport.md[diff]
• services/network/public/cpp/features.cc[diff]
• services/network/public/cpp/features.h[diff]
• services/network/shared_dictionary/shared_dictionary_network_transaction.cc[diff]
• services/network/shared_dictionary/shared_dictionary_network_transaction_unittest.cc[diff]
• testing/variations/fieldtrial_testing_config.json[diff]
• tools/metrics/histograms/enums.xml[diff]