With this change, IASetVertexBuffers is only called from a single
code site in StateManager11. All other classes call through to here.
This will make adopting the dirty bits for InputLayouts and VBs much
simpler.

BUG=angleproject:2052

Change-Id: I6e7b6146deb7f80f5f0e75dd0aff18eb4cc2bfc2
Reviewed-on: https://chromium-review.googlesource.com/524232Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>