media: Don't set use_vaapi to true on non-X11.

This is needed so that use_vaapi is false on Chromecast. Before this CL,
this was causing troubles with a build [1, 2].

[1] https://chromium-review.googlesource.com/c/chromium/src/+/2578836
[2] https://ci.chromium.org/ui/p/chromium/builders/ci/Cast%20Audio%20Linux/92923/overview

Bug: b:173167846
Test: None
Change-Id: I5d33790d81e50f099decc1ae03c159a3dc575dcf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2580449
Reviewed-by: Ted Meyer <tmathmeyer@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Andres Calderon Jaramillo <andrescj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834923}

media/gpu/args.gni

@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 import("//build/config/chromeos/ui_mode.gni")
+import("//build/config/ui.gni")
 
 declare_args() {
   # Indicates if V4L plugin is used.
@@ -20,7 +21,8 @@ declare_args() {
   # is typically the case on x86-based ChromeOS devices.
   # VA-API should also be compiled by default on x11-using linux devices
   # using x86/x64.
-  use_vaapi = is_linux && (current_cpu == "x86" || current_cpu == "x64")
+  use_vaapi =
+      is_linux && use_x11 && (current_cpu == "x86" || current_cpu == "x64")
 
   # Indicates if ChromeOS protected media support exists. This is used
   # to enable the CDM daemon in Chrome OS as well as support for