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This mostly doesn't work, so also add some documentation that tells
people not to use it. (The documentation also has some other
information.)

No behavior change for regular builds.

Bug: 1149587
Change-Id: I03f4f5b65a0d010636a499b043bfcf31b0cd34b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2574601Reviewed-by: Hans Wennborg <hans@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834446}