I came across this today while reviewing something else.

The current signature is this:
  CheckOverlayCapabilities(gfx.mojom.AcceleratedWidget widget,
      array<ui.ozone.mojom.OverlaySurfaceCandidate> candidates) =>
          (gfx.mojom.AcceleratedWidget widget,
          array<ui.ozone.mojom.OverlaySurfaceCandidate> candidates,
          array<ui.ozone.mojom.OverlayStatus> status);

It's a bit odd to echo |candidate| through IPC when the caller could just make a copy and bind it directly into the return callback if desired.

One other potential issue: it looks like |candidates| and |status| should match in size. But I'm not sure that anything actually enforces that (though I'm not sure if this can lead to actual bugs either)