D3D11: Cache element array buffer updates. This attempts to reduce the amount of redundant validation done between indexed draw calls. It keeps the cached info in the VertexArray11 class. It also includes a fix to a missing direct buffer invalidation in CopyBufferSubData which was turning up with the new caching. Reduces overhead in the D3D11 indexed rendering perf test such that it leads to an increased score of about 20%. BUG=angleproject:2229 Change-Id: I63121bea19a9c8198e1925ed6a1460838e8f8955 Reviewed-on: https://chromium-review.googlesource.com/765262 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Yuly Novikov <ynovikov@chromium.org>