Also fixes an issue in buffer copy synchronization.

Bug: angleproject:2645
Change-Id: Ibca7052daaf1e6fe37913c8a8216ec33c66426b6
Reviewed-on: https://chromium-review.googlesource.com/1096911Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Luc Ferron <lucferron@chromium.org>