GLSyncs created during MEC were getting garbage for the high order
bits when when run on 64-bit applications. That's because the
iterator is 32-bit and was being bitcast to 64-bit. This change
zero extends the iterator before converting to a GLSync.
Test: Lots of traces were hitting this
Bug: angleproject:4048
Change-Id: Ia22da945e014de0caad2a3663a33f996bb781b19
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2486403Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Cody Northrop <cnorthrop@google.com>
