The check for "IsPlatformAvailable" was inverted. We were incorrectly
trying only to create a context on invalid configs.

Bug: angleproject:4449
Change-Id: I03fbdc7794a338a55b9dc64521915a8be247bf35
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2376045
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>