D3D: Add streaming buffer initialize. This gives us better control over the error returned by the init routine. Also changes the streaming buffer in VertexDataManager to not be a unique_ptr. Bug: angleproject:2738 Change-Id: I3193840dfb71c7574adbe65bc5f9227f4add1fd3 Reviewed-on: https://chromium-review.googlesource.com/1151448 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>