This makes "Data" a proper class, and enforces access control when used in FramebufferImpl. This gives a cleaner refactor when we switch the internals of the class to use value types to store attachments instead of pointer types. BUG=angleproject:963 Change-Id: If825095458eaf9367f616f0bb54084025efb9882 Reviewed-on: https://chromium-review.googlesource.com/265937Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Brandon Jones <bajones@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>
