Refactor BufferStorage11. In preparation for transform feedback, BufferStorage11 needs to be able to handle a non-staging buffer being updated. Each D3D11 buffer now has an incrementing data revision associated with it so the most up-to-date buffer is always known. Staging buffers are now represented like any other DirectBuffer. Change-Id: I6e881867cb2bd02d600213d08cce3ebba316c525 Reviewed-on: https://chromium-review.googlesource.com/184395 Tested-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Shannon Woods <shannonwoods@chromium.org>