CLIENT_MAPPED_BUFFER_BARRIER_BIT_EXT requires a memory barrier: shader
buffer write -> host read.  According to the spec, the data is only
available after a call to glFinish or wait on sync:

> The application must call MemoryBarrier with the
> CLIENT_MAPPED_BUFFER_BARRIER_BIT_EXT set and then call FenceSync with
> SYNC_GPU_COMMANDS_COMPLETE (or Finish). Then the CPU will see the
> writes after the sync is complete.

When a buffer is written to by the GPU, ANGLE calls
onHostVisibleBufferWrite(), which ensures a "memory write -> host read"
barrier is issued at the end of the command buffer.

Additionally, persistently mapped buffers use
VK_MEMORY_PROPERTY_HOST_COHERENT_BIT, so there's no need for a call to
vkInvalidateMappedMemoryRanges.

As a result, there's nothing necessary in ANGLE to do for this barrier
bit.  Note that should persistenly mapped buffers start using
non-coherent memory, this barrier should imply a call to
vkInvalidateMappedMemoryRanges for the persistently mapped buffers.

Bug: angleproject:5070
Change-Id: Iaeae019dadfa659a47d2dac41c0c09f1c15e584b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2689380
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Charlie Lao <cclao@google.com>