ozone: Call glFlush before submitting a buffer.

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

Index: ui/ozone/platform/drm/gpu/gbm_surfaceless.cc
diff --git a/ui/ozone/platform/drm/gpu/gbm_surfaceless.cc b/ui/ozone/platform/drm/gpu/gbm_surfaceless.cc
index 4b4913b78f7cf8d7b718e263c47cb9ca62a820b2..7c3cba72064eaf07345776b56d1914d754098089 100644
--- a/ui/ozone/platform/drm/gpu/gbm_surfaceless.cc
+++ b/ui/ozone/platform/drm/gpu/gbm_surfaceless.cc
@@ -103,6 +103,9 @@ void GbmSurfaceless::SwapBuffersAsync(const SwapCompletionCallback& callback) {
     return;
   }
 
+  // TODO(dcastagna): remove glFlush since eglImageFlushExternalEXT called on
+  // the image should be enough (crbug.com/720045).
+  glFlush();
   unsubmitted_frames_.back()->Flush();
 
   SwapCompletionCallback surface_swap_callback = base::Bind(