Remove return bool in SurfaceOzoneEGL::OnSwapBuffersAsync

ui/ozone/platform/drm/gpu/gbm_surfaceless.h

Index: ui/ozone/platform/drm/gpu/gbm_surfaceless.h
diff --git a/ui/ozone/platform/drm/gpu/gbm_surfaceless.h b/ui/ozone/platform/drm/gpu/gbm_surfaceless.h
index 54f749e96cc7512379af522c2be6d272da47c2ff..99abc24e9853caede3f6267340c91af8b04021fb 100644
--- a/ui/ozone/platform/drm/gpu/gbm_surfaceless.h
+++ b/ui/ozone/platform/drm/gpu/gbm_surfaceless.h
@@ -36,7 +36,7 @@ class GbmSurfaceless : public SurfaceOzoneEGL {
   intptr_t GetNativeWindow() override;
   bool ResizeNativeWindow(const gfx::Size& viewport_size) override;
   bool OnSwapBuffers() override;
-  bool OnSwapBuffersAsync(const SwapCompletionCallback& callback) override;
+  void OnSwapBuffersAsync(const SwapCompletionCallback& callback) override;
   scoped_ptr<gfx::VSyncProvider> CreateVSyncProvider() override;
   bool IsUniversalDisplayLinkDevice() override;