This ensures that when the global texture manager is released, there is a valid
context.

BUG=angleproject:1639

Change-Id: I1b75885e9dc02b607bb1a386de394f6087429f5d
Reviewed-on: https://chromium-review.googlesource.com/442074Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>