Disable use_system_{libdrm,minigbm} by default on Chromecast

This broke the build of some Nest devices that don't have libdrm in the
build environment. Partially revert enabling this on Linux to fix the
build.

BUG=1031269
TBR=thomasanderson

Change-Id: I596f30849da48d44c246983a08a5cd08f2fbff08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1976194Reviewed-by: Michael Spang <spang@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726426}

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/chromecast_build.gni")
 import("//build/config/chromeos/args.gni")
 import("//build/config/linux/pkg_config.gni")
 
@@ -9,8 +10,8 @@ assert(is_linux)
 declare_args() {
   # Controls whether the build should use the version of libdrm library shipped
   # with the system. In release builds of desktop Linux and Chrome OS we use the
-  # system version. Chromecast will use this as well.
-  use_system_libdrm = is_chromeos_device || is_desktop_linux
+  # system version. Some Chromecast devices use this as well.
+  use_system_libdrm = is_chromeos_device || (is_desktop_linux && !is_chromecast)
 }
 
 if (use_system_libdrm) {

third_party/minigbm/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/chromecast_build.gni")
 import("//build/config/linux/pkg_config.gni")
 
 assert(is_linux)
@@ -10,7 +11,7 @@ declare_args() {
   # Controls whether the build should use the version of minigbm library shipped
   # with the system. In release builds of desktop Linux and Chrome OS we use the
   # system version.
-  use_system_minigbm = is_desktop_linux
+  use_system_minigbm = is_desktop_linux && !is_chromecast
 
   use_amdgpu_minigbm = false
   use_exynos_minigbm = false