GitLab

. 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.

BUG=none

Review-Url: https://codereview.chromium.org/2801943002
Cr-Commit-Position: refs/heads/master@{#463049}