This ASSERT was being overly restrictive, when the code would work
correctly when reading from system memory storage as well as staging.

BUG=angleproject:1555

Change-Id: Ib45185d8c127a1fb2ce54db9e96fe41d68d5ff2e
Reviewed-on: https://chromium-review.googlesource.com/402269Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>