Fix GpuChannelHost destruction and races

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
Committed: https://crrev.com/765e52870520c6e60d913c307577c1d1fec3a381
Cr-Commit-Position: refs/heads/master@{#328917}

[piman, 2015-05-07 22:04:45]

piman@chromium.org changed reviewers:
+ jbauman@chromium.org

[piman, 2015-05-07 22:04:45]

This probably interacts with your https://codereview.chromium.org/1117183006/
but we should be able to resolve conflicts realtively easily.

[jbauman, 2015-05-07 23:31:10]

lgtm

[piman, 2015-05-08 01:29:47]

The CQ bit was checked by piman@chromium.org

[commit-bot: I haz the power, 2015-05-08 01:31:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1128233004/1

[commit-bot: I haz the power, 2015-05-08 03:43:12]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-08 03:44:12]

Patchset 1 (id:??) landed as
https://crrev.com/765e52870520c6e60d913c307577c1d1fec3a381
Cr-Commit-Position: refs/heads/master@{#328917}