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Updated per go/cros-ppi-spectrum. [270-310) now uses 2.4 and
use 2.2525.. only for Slate which has 3000x2000 panel.
No change to ARC++ scale for this range.

2.4 * 1600 produces small error (0.00024). Make sure it's
within the epsilon used in cc. (0.001) so that the screenshot size
is correct (See DisplayChangeObserverTest.FindDeviceScaleFactor)

Bug: 1070828
Test: covered by unittests. manually tested on yoga and nocturne
Change-Id: I2899fd774fcbd461231282b97fecead6a308b949
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2397321
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807774}