Revert "gl: Disable swiftshader on audio only devices"

This reverts commit cb7925c3.

Reason for revert: Seems to be the likely cause of failures starting here: https://ci.chromium.org/p/chromium/builders/ci/Cast%20Audio%20Linux/50083

Speculatively reverting.

Original change's description:
> 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/+/1752167
> Reviewed-by: Kenneth Russell <kbr@chromium.org>
> Commit-Queue: Michael Spang <spang@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#686505}

TBR=spang@chromium.org,kbr@chromium.org

Change-Id: I69eef9416529cfcff37e5f657c855908e453153b
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: b/139101843
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1752911Reviewed-by: Tommy Li <tommycli@chromium.org>
Commit-Queue: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686595}

ui/gl/BUILD.gn

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

ui/ozone/platform/headless/BUILD.gn

@@ -31,7 +31,6 @@ 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

@@ -20,7 +20,6 @@
 #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"
@@ -175,11 +174,7 @@ base::FilePath HeadlessSurfaceFactory::GetPathForWidget(
 
 std::vector<gl::GLImplementation>
 HeadlessSurfaceFactory::GetAllowedGLImplementations() {
-  return std::vector<gl::GLImplementation> {
-#if BUILDFLAG(ENABLE_SWIFTSHADER)
-    gl::kGLImplementationSwiftShaderGL,
-#endif
-  };
+  return std::vector<gl::GLImplementation>{gl::kGLImplementationSwiftShaderGL};
 }
 
 GLOzone* HeadlessSurfaceFactory::GetGLOzone(