No functional change.
When we add vertex data format conversion to Vulkan we will need to be
able to return an error from VertexArray::syncState().

BUG=angleproject:2405

Change-Id: I4b537946ecbb6593280b6510c5cd8d8e3c65e8dd
Reviewed-on: https://chromium-review.googlesource.com/982897
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Luc Ferron <lucferron@chromium.org>