Fix GpuChannelHost destruction and races

      https://codereview.chromium.org/583043005 introduced
GpuChannelHost::DestroyChannel but it introduced a new state to GpuChannelHost
which was not captured everywhere and had races.

1- Ensure GpuChannelHost::DestroyChannel is always called on the main thread, before destruction.
2- properly handle the channel_ == NULL state.
3- Fix races (channel_ is used on any thread on windows, must be locked)

Also make sure contexts are destroyed before the channel, for sanity.

BUG=451456,471822

Review URL: https://codereview.chromium.org/1128233004

Cr-Commit-Position: refs/heads/master@{#328917}