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It existed for Windows, and we currently don't do WPO on Windows.
When we bring it back, it'll likely be different from /GL cl.exe-style
WPO.

Depends on https://chromium-review.googlesource.com/c/chromium/third_party/ffmpeg/+/2065789

Bug: 1053958,598772
Change-Id: Ie2ea04ad54f0d3b6711ffb7682fe1ae15662cb95
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2066106
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743283}