Vulkan: flush StreamingBuffer before unmap.
Vulkan: flush StreamingBuffer before unmap.

Unmap does not flush, so we need to do it explicitly.

BUG=angleproject:2436

Change-Id: Id102825b1329d97965f07b97c227214fa7524a0e
Reviewed-on: https://chromium-review.googlesource.com/980927
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Luc Ferron <a2389c1bcb3ee32b089866e04e35d1a302020674@chromium.org>
Commit-Queue: Frank Henigman <4f3ccfa167f952ff7263448ff4377acff6a23e5c@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle