This will enable the next frame to render while the previous frame's
command buffers are executing and rendering is happening. This should
speed up the Vulkan perf test.

BUG=angleproject:1898

Change-Id: I0820300d7934ef280cf75628239fe40dc96b8242
Reviewed-on: https://chromium-review.googlesource.com/445799
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>