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There is existing documentation about the details, but as yet no overall
"what is this thing" summary.

Change-Id: I057d1c67a8f7d5fe1cbbeb65505a3be0419bfafc
Reviewed-on: https://chromium-review.googlesource.com/1246951
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594696}