Set the use_system_libdrm 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: I8191fe2a2a6d910ceeec8fba90c43f199a753ce9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1963135Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#724072}

build/config/linux/libdrm/BUILD.gn

 # Copyright 2018 The Chromium Authors. All rights reserved.
 # 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/linux/pkg_config.gni")
 
 assert(is_linux)
@@ -11,7 +12,7 @@ declare_args() {
   # use the system version, but when building on dev workstations we
   # bundle it because Ubuntu doesn't ship a usable version.
   # Chromecast will use this as well.
-  use_system_libdrm = false
+  use_system_libdrm = is_chromeos_device
 }
 
 if (use_system_libdrm) {