If a SSBO is initialized with data, a staging buffer was created. When the
data was read back, it was read from the staging buffer instead of the GPU.
This patch fixes that by making the UAV buffer the latest buffer.

Bug: angleproject:2990
Test: angle_end2end_tests
Change-Id: I43c8c85144c74cc9b317577b7c834e3f7e347f72
Reviewed-on: https://chromium-review.googlesource.com/c/1359712Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Bryan Bernhart <bryan.bernhart@intel.com>
Commit-Queue: Bryan Bernhart <bryan.bernhart@intel.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>