cc::TargetColorParams: Store HDR headroom

Once upon a time, it was thought that the SDR white level would matter
for HDR images. The idea was that HDR images would render the same as
HDR video on Windows.

The way HDR video renders on Windows is a dying bad idea, and will never
be allowed to pollute images. Before we came to this enlightened
position, we plumbed the SDR white level into cc::TargetColorParams, and
expressed HDR headroom in linear form.

Remove SDR white.

Change HDR headroom to be in log2, and also make it optional. The
optional-ness will be relevant when we add the canvas API to tone map to
a specific HDR headroom (the target color space is determined by the
image provider, and the target headroom can be overridden by the DrawOp
-- it is awkward and will continue to be awkward).

Bug: 428575083
Change-Id: I093d311d732cd544a284b3cf26004693c5e2d6b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6797010
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: ccameron chromium <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1493981}