This test is failing because our out-of-date Windows 7 NV bots
can't create a GLES 3.0 context. Skip this test until the machines
are updated.

Also updates our wrapper classes to check for an empty handle
before calling delete. Prevents a crash trying to call null functions.

Bug: chromium:1090139
Change-Id: I0bb58f8bd167691cd241fb0df30cdf4b2f6adf5b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2225447
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>