ozone: fix crash when running video decode unittests.

In GLSurfaceOzoneSurfacelessSurfaceImpl::Destroy, if the previous
context was NULL, do not make it current.

BUG=chromium:602921
TEST=Run vda unittests on oak.

Review URL: https://codereview.chromium.org/1887563002

Cr-Commit-Position: refs/heads/master@{#386988}
bool GLSurfaceOzoneSurfaceless::PendingFrame::ScheduleOverlayPlanes(
    gfx::AcceleratedWidget widget) {
  for (const auto& overlay : overlays)
    if (!overlay.ScheduleOverlayPlane(widget))
      return false;
  return true;
}

bool GLSurfaceOzoneSurfaceless::PendingFrame::ScheduleOverlayPlanes(
    gfx::AcceleratedWidget widget) {
  for (const auto& overlay : overlays)
    if (!overlay.ScheduleOverlayPlane(widget))
      return false;
  return true;
}
