gl: Disable swiftshader on audio only devices

Audio only devices have no display and no GPU. Disable swiftshader so we
don't see an error on startup trying to load it.

Bug: b/139101843

Change-Id: Ib15d38a1da17e8a5057390879fd684a8b2b9bd67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1752167Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686505}

ui/gl/BUILD.gn

@@ -4,6 +4,7 @@
 
 import("//build/buildflag_header.gni")
 import("//build/config/chrome_build.gni")
+import("//build/config/chromecast_build.gni")
 import("//build/config/jumbo.gni")
 import("//build/config/linux/pkg_config.gni")
 import("//build/config/ui.gni")
@@ -13,7 +14,7 @@ import("//ui/ozone/ozone.gni")
 
 declare_args() {
   enable_swiftshader = (is_win || is_linux || (is_mac && use_egl) ||
-                        is_chromeos || is_fuchsia) &&
+                        is_chromeos || is_fuchsia) && !is_cast_audio_only &&
                        (target_cpu == "x86" || target_cpu == "x64" ||
                         target_cpu == "arm" || target_cpu == "arm64" ||
                         target_cpu == "mipsel" || target_cpu == "mips64el")

ui/ozone/platform/headless/BUILD.gn

@@ -31,6 +31,7 @@ source_set("headless") {
     "//ui/events/ozone:events_ozone_layout",
     "//ui/events/platform",
     "//ui/gfx/geometry",
+    "//ui/gl:buildflags",
     "//ui/ozone:ozone_base",
     "//ui/ozone/common",
     "//ui/platform_window",

ui/ozone/platform/headless/headless_surface_factory.cc

@@ -20,6 +20,7 @@
 #include "ui/gfx/native_pixmap.h"
 #include "ui/gfx/skia_util.h"
 #include "ui/gfx/vsync_provider.h"
+#include "ui/gl/buildflags.h"
 #include "ui/gl/gl_surface_egl.h"
 #include "ui/ozone/common/egl_util.h"
 #include "ui/ozone/common/gl_ozone_egl.h"
@@ -174,7 +175,11 @@ base::FilePath HeadlessSurfaceFactory::GetPathForWidget(
 
 std::vector<gl::GLImplementation>
 HeadlessSurfaceFactory::GetAllowedGLImplementations() {
-  return std::vector<gl::GLImplementation>{gl::kGLImplementationSwiftShaderGL};
+  return std::vector<gl::GLImplementation> {
+#if BUILDFLAG(ENABLE_SWIFTSHADER)
+    gl::kGLImplementationSwiftShaderGL,
+#endif
+  };
 }
 
 GLOzone* HeadlessSurfaceFactory::GetGLOzone(