On some drivers, the WGL_ARB_create_context_robustness extension is present but
the created context does not expose the extensions.  Be more defensive and only
expose robustness if we're able to create contexts that have it.

BUG=angleproject:1538

Change-Id: I3d02b2071c7b2c576abe66cc283cee467e400fd3
Reviewed-on: https://chromium-review.googlesource.com/391833Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: John Bauman <jbauman@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>