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}
gfx::SwapResult GLSurfaceOzoneSurfaceless::PostSubBuffer(int x,
                                                         int y,
                                                         int width,
                                                         int height) {
  NOTREACHED();
  return gfx::SwapResult::SWAP_FAILED;
}

gfx::SwapResult GLSurfaceOzoneSurfaceless::PostSubBuffer(int x,
                                                         int y,
                                                         int width,
                                                         int height) {
  NOTREACHED();
  return gfx::SwapResult::SWAP_FAILED;
}
