Set the use_evdev_gestures GN arg to true when building for CrOS.

This arg never changes among all supported CrOS device builds of Chrome.
By giving it a proper default val in those builds, we can reduce the amount
of inputs needed when building for CrOS (ie: simple chrome & chromeos-chrome
ebuild).

Bug: 937821
Change-Id: If370ec73383201f80efc9a7e97c73f07739ddcb3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1963097Reviewed-by: Michael Spang <spang@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#724064}

ui/events/ozone/evdev/BUILD.gn

@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/chromeos/args.gni")
 import("//build/config/ui.gni")
 
 assert(use_ozone && is_linux)
@@ -10,7 +11,7 @@ visibility = [ ":*" ]
 
 declare_args() {
   # Support ChromeOS touchpad gestures with ozone.
-  use_evdev_gestures = false
+  use_evdev_gestures = is_chromeos_device
 }
 
 config("evdev_config") {