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This adds a side-channel through which a gfx::HDRMetadata can be
attached to an IOSurface. In practice this can be written
by the software decoder in the renderer process and read by the
compositor in the GPU process.

Bug: 1101041
Change-Id: I36a8f0cc8562fe1daa2e3e4c131aa7f305644aa5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2531655
Commit-Queue: ccameron <ccameron@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827187}