If we are restoring a lost device we should have deleted all Contexts.
This would have freed any bound textures.

Remove the release call and replace it with an ASSERT. One usage of the
Proxy Context remains: Display::destroyImage.

Bug: angleproject:2714
Change-Id: Id3e4b7b0341092a8fae37705a6bb171167196fe2
Reviewed-on: https://chromium-review.googlesource.com/1137880
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>