egl: Don't query EGL_POST_SUB_BUFFER_SUPPORTED_NV without extension

egl: Don't query EGL_POST_SUB_BUFFER_SUPPORTED_NV without extension

We should only query the surface for this feature if the requisite
extension is supported. Querying has incorrect results with llvmpipe
on ubuntu trusty, which doesn't support the extension.

BUG=440473
TEST=chrome --use-gl=egl (chromeos build) - no partial swap errors

Committed: https://crrev.com/9d1c8ff57b7042918cc5f81b2cd37d974df2e557
Cr-Commit-Position: refs/heads/master@{#307588}

[spang, 2014-12-09 19:31:20]

spang@chromium.org changed reviewers:
+ kbr@chromium.org

[Ken Russell (switch to Gerrit), 2014-12-09 22:18:49]

lgtm

[spang, 2014-12-09 22:53:29]

The CQ bit was checked by spang@chromium.org

[commit-bot: I haz the power, 2014-12-09 22:53:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/792623002/1

[commit-bot: I haz the power, 2014-12-09 23:57:25]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-12-09 23:58:17]

Patchset 1 (id:??) landed as
https://crrev.com/9d1c8ff57b7042918cc5f81b2cd37d974df2e557
Cr-Commit-Position: refs/heads/master@{#307588}