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Since ARC R, ARC started to natively support display density. That means
now one Android pixel always matches the display's physical pixel, and
will not be affected by default_dsf which can dynamically change when
the screen scaling is changed by the user.

This CL fixes the issue by adding GetDeviceScaleFactorForWindow to
WMHelper. It returns the same value as GetDefaultDeviceScaleFactor()
when |default_scale_cancellation| is true i.e. ARC not supporting native
density. Otherwise, it returns the display's density.

Additionally, |default_scale_cancellation| is forwarded from
WaylandRemoteShell to WMHelper.

TEST=manual
BUG=b:172031529

Change-Id: I04c2a14712c49760a38c2e21eaf4a2244195397d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2525703Reviewed-by: Yuichiro Hanada <yhanada@chromium.org>
Reviewed-by: Michael Spang <spang@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827167}