In some cases we could end up with bufferSubData updates being left
in staging buffers during a draw call. Currently we rely on 
InputLayoutCache::applyVertexBuffers to flush all pending buffer
updates, but this could be done in VertexArray11::syncState, or
even in the draw call sync state.

BUG=angleproject:1156
BUG=chromium:759403

Change-Id: I7019425f874414bf5857b825ddae7e022750c759
Reviewed-on: https://chromium-review.googlesource.com/642057Reviewed-by: Jamie Madill <jmadill@chromium.org>