This change implements timeline fences in Dawn.
It includes methods and descriptor members to eventually
support multi-queue, but does not implement them.
Bug: dawn:26
Change-Id: I81d5fee6acef402fe099227a034d9669a89ab6c3
Reviewed-on: https://dawn-review.googlesource.com/c/2460
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
