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>