Enable SwiftShader fallback on desktop Linux.

ui/gl/BUILD.gn

 # Copyright 2014 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/buildflag_header.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/linux/pkg_config.gni")
 import("//build/config/ui.gni")
 import("//ui/ozone/ozone.gni")
 import("//testing/test.gni")
 
 declare_args() {
-  enable_swiftshader = is_win
+  enable_swiftshader = (is_win || (is_linux && !is_chromeos && use_x11)) &&
+                       (target_cpu == "x86" || target_cpu == "x64")
 }
 
 use_egl = is_win || is_android || is_linux
 use_glx = use_x11 || ozone_platform_x11
 
 if (is_android) {
   import("//build/config/android/config.gni")
   import("//build/config/android/rules.gni")
 }
 
@@ skipping 427 matching lines @@
     sources = [
       "../android/java/src/org/chromium/ui/gl/SurfaceTextureListener.java",
       "../android/java/src/org/chromium/ui/gl/SurfaceTexturePlatformWrapper.java",
     ]
     public_deps = [
       ":surface_jni_headers",
     ]
     jni_package = "ui/gl"
   }
 }