D3D11: Consolidate SRV allocation. This change also moves all SRV storage to use the custom smart pointer classes in ResourceManager11. Every single SRV is now allocated in ResourceManager11. For TextureStorage11 and RenderTarget11, we use a shared pointer type since the SRVs use shared ownership by both classes. BUG=angleproject:2034 Change-Id: I90666fbbbc23ff10870b7401f3da09ef79258136 Reviewed-on: https://chromium-review.googlesource.com/503250 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>