This workaround avoids using inter-buffer copies involving constant
buffers. Instead of copying from a staging storage to a constant
buffer (or vice-versa), the Buffer memory managment will keep the
system memory storage around permanently. This uses a bit more memory
but should fix all the problematic cases where we have the canonical
version of the buffer data inside GPU memory.

BUG=chromium:593024

Change-Id: I2ce0cfd22784aa43e819ce2df2d0da540d0a0cfe
Reviewed-on: https://chromium-review.googlesource.com/412140Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>