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This will be used by a D-Bus service to allow developers to tweak
gesture properties at run-time. See
go/cros-gesture-properties-dbus-design (Google internal only, sorry) for
more details about the full design.

Some of this code is based on Jacek Fedoryński's
https://crrev.com/c/1350982.

Bug: 952054
Test: As part of a series, tested that the D-Bus service can get and set
properties. Alone, checked that Chromium and unit tests build.

Change-Id: Icf0e82e02b5aa6dcdc4efc258292fc63d208e4e1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1589445
Commit-Queue: Harry Cutts <hcutts@chromium.org>
Reviewed-by: Michael Spang <spang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665326}