This will ensure we don't destroy the image views when they are still
in use by other Contexts.

Bug: angleproject:2464
Change-Id: I1d3ba2ad241250e31ea32873446c4cb23971750d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1843236Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>