D3D11: Move CreateStagingTexture to Renderer11. This will make staging texture tracking easier. BUG=angleproject:2034 Change-Id: I2f0418cc5784100bf6fba59b4bda1b2dd9dc18d1 Reviewed-on: https://chromium-review.googlesource.com/503251 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>