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::Initialize(gfx::GLSurface::Format format) {
  if (!SurfacelessEGL::Initialize(format))
    return false;
  vsync_provider_ = ozone_surface_->CreateVSyncProvider();
  if (!vsync_provider_)
    return false;
  return true;
}

bool GLSurfaceOzoneSurfaceless::Initialize(gfx::GLSurface::Format format) {
  if (!SurfacelessEGL::Initialize(format))
    return false;
  vsync_provider_ = ozone_surface_->CreateVSyncProvider();
  if (!vsync_provider_)
    return false;
  return true;
}
