This reverts commit 82a420bb.

Reason for revert: breaks readPixels on Android, probably because memory there is not host coherent by default.

Original change's description:
> Vulkan: readPixels improvement - use DynamicBuffer
> 
> - Instead of using a new Staging image everytime, we now
> just use a DynamicBuffer that should be bit less wasteful.
> 
> Bug: angleproject:2480
> 
> Change-Id: I09edfb16f6ec49a0ca320d9e5dd9ce34e9dbf99e
> Reviewed-on: https://chromium-review.googlesource.com/1059452
> Commit-Queue: Luc Ferron <lucferron@chromium.org>
> Reviewed-by: Jamie Madill <jmadill@chromium.org>
> Reviewed-by: Geoff Lang <geofflang@chromium.org>

TBR=geofflang@chromium.org,jmadill@chromium.org,lucferron@chromium.org

Change-Id: Id7c493915ec7131f5202a7224267a79cd53007d2
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: angleproject:2480
Reviewed-on: https://chromium-review.googlesource.com/1064770Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>