To support indexed draws it's useful to have a cheap query for the
presence of vertex attribute data in client memory.
This patch adds a bit mask to keep track of such attributes.
It also lets us simplify VertexArrayVk::streamVertexData() slightly.

BUG=angleproject:1683

Change-Id: I871bfb885112650b025e110c383db3c391eafb90
Reviewed-on: https://chromium-review.googlesource.com/947927
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>