The spec refers to Sync objects, FenceSyncs being a subtype. The
motivation for this fix is to clear up the FenceSync_ entry point for
auto-generation.

BUG=angleproject:1309

Change-Id: I94c440476d701628575e7a3eea68b6dd110f41c3
Reviewed-on: https://chromium-review.googlesource.com/636516Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>