The read framebuffer may not be complete and be incapable of syncing.

Removed the generate syncDirtyObjects method so each caller must make sure they
are only syncing objects that are known to be valid for the operation.

BUG=angleproject:2737

Change-Id: Ia8edf3fca3a8369aa813be46ba99f6b50a36b2e6
Reviewed-on: https://chromium-review.googlesource.com/1151621
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>