gpu: Some code cleanups.
. Remove unused BrowserGpuChannelHostFactory::AddFilterOnIO().
. Use a weak-ptr to bind callbacks to GpuProcessHost, instead of looking
up the GpuProcessHost from the host-id (using GpuProcessHost::FromID()).
. Remove some null checks for |gpu_channel_manager_| in GpuService,
because it is created during initialization, and destroyed only in the
dtor. None of the other callbacks should trigger before initialization.
So the null checks should not be necessary.