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This is needed as a prerequisite for [1].  It also improves performance
a bit by replacing a node-based data structure with a flat one.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/2622521

Change-Id: Ibe2e522f6c131876ed73793305524c25b42ab910
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2625784
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844303}