This also adds a test for updating a Texture that is in-use. This will
ensure our Texture updates occur at the right time when we're
implementing command re-ordering.

Bug: angleproject:2264
Bug: angleproject:2200
Change-Id: Id6040d7238eca031e3cc7b27564d8ea815bf3d73
Reviewed-on: https://chromium-review.googlesource.com/801031
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>