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list -> set, O(n^2) -> O(n).
Timing, without this patch (on a typical trace from a device):
real	1m49.774s
user	1m49.394s
sys	0m0.316s

With:
real	0m5.607s
user	0m5.482s
sys	0m0.120s

BUG=448054

Review URL: https://codereview.chromium.org/873003003

Cr-Commit-Position: refs/heads/master@{#313513}