<chromium> Only enable robustness if the shared context is using it

Chromium currently assumes that all GLContexts are checking the availability
of robustness on the system through the same code path. Since we're passing
it a context created by Qt, and that Qt never uses
EGL_EXT_create_context_robustness, the context creation fails when trying to
share a non-robustness-enabled context with one that does.

Fix the issue by asking the shared GLContext if it was allocated using the
extension, Chromium's contexts currently always return true and the Qt context
will return false. This relies on the fact that the Qt context is always the
first one added to the ShareGroup and that it is going to be the one queried,
but this should be reasonably reliable.

Change-Id: I6456429db678f077cd8a72032711361da01e72ea
Reviewed-by: Andras Becsi <andras.becsi@digia.com>