Support multiple contexts in command_buffer_gles2

Support multiple contexts in command_buffer_gles2

Fix problems with multiple contexts when using command_buffer_gles2
library.

Rendering to two different command_buffer_gles2-provided contexts would
not work, as the last created native context would remain current.

Also fixes a problem with Skia, where Skia would create and use a GLX
context directly, as well as using command_buffer_gles2 context.

BUG=skia:2992
Committed: https://crrev.com/213e490b733180b643600217e0fdc39e596b109a
Cr-Commit-Position: refs/heads/master@{#356058}

[Kimmo Kinnunen, 2015-10-26 09:38:52]

kkinnunen@nvidia.com changed reviewers:
+ hendrikw@chromium.org, piman@chromium.org

[piman, 2015-10-26 15:59:41]

lgtm

[hendrikw, 2015-10-26 16:14:13]

On 2015/10/26 15:59:41, piman (slow to review) wrote:
> lgtm

Awesome, thanks!  lgtm as well.

[hendrikw, 2015-10-26 16:15:39]

The CQ bit was checked by hendrikw@chromium.org

[commit-bot: I haz the power, 2015-10-26 16:15:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1416043005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1416043005/1

[commit-bot: I haz the power, 2015-10-26 16:26:09]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-26 16:27:03]

Patchset 1 (id:??) landed as
https://crrev.com/213e490b733180b643600217e0fdc39e596b109a
Cr-Commit-Position: refs/heads/master@{#356058}