Serializes buffers' states stored on CPU.
Gets buffers contents on GPU by mapping buffers to CPU's address space,
then copy their data. Unmap buffers after finish. Since this feature is
for tests only, it is only implemented for the Vulkan backend.

Adds buffer serialization to serializeContext method so that capture
replay regresssion testing now compares the states of buffers too.

Bug: angleproject:4817
Change-Id: Ic9b529701014d5ba8420023a021cd5ea381bd9a1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2295950
Commit-Queue: Manh Nguyen <nguyenmh@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>