gpu:: Remove GpuDataManagerObserver::OnVideoMemoryUsageStatsUpdate().
TaskManagerImpl is the only observer which requires this callback. In
order to get the video memory usage, TaskManagerImpl installs itself as
an observer, makes the request through GpuDataManager, and waits for the
observer callback to be called. Instead, change this so the request to
GpuDataManager::RequestVideoMemoryUsageStatsUpdate() includes the
callback that needs to run when the stats become available.