With the new resource tracking scheme the CommandGraph, tracking a
"Context serial" aka "current" serial is no longer necessary for
CommandGraphResources. Serial tracking has been moved to the shared
ResourceUse struct that gets updated on a command submission. Thus
we don't need to store the serial as a current separate piece of info
in BufferHelper/ImageHelper.

Will lead to further redesign for the multi-threading support for
Vulkan.

Bug: angleproject:2464
Change-Id: I1ae4bcc27fcfb93422b4ab4c9682a458e482f295
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1785990
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>