D3D: Make directStoragePossible a local static method. This method doesn't need to call through to the VertexBuffer class, since in only really depends on vertex format caps. This makes the code local to the VertexDataManager, the only place it is used. Refactoring patch only, for VertexArray11 dirty bits. BUG=angleproject:1327 Change-Id: I315313a72a00186930d7f9b1091ccb91f37f9f96 Reviewed-on: https://chromium-review.googlesource.com/329740 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Zhenyao Mo <zmo@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>