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Local profiling suggests that converting std::set to std::vector will save
around 10% of the overall startup time, likely due to the reduced allocs
involved with std::vector.

This change also reduces lookups by traversing the incoming dictionary
and iterating on the available values instead of looking through all
values for all features.

Finally, cleaned up and enforced the testing accessors via code and friend
declarations

BUG=460987

Review URL: https://codereview.chromium.org/1010973013

Cr-Commit-Position: refs/heads/master@{#322609}