This cleans up some of the logic around command completion. We no longer
query each Fence in order after a finishToSerial call. Instead we clear
out all the commands that have been completed up to the latest Fence.

Also cleans up ordering code. Because we now have a linear ordering of
Serials we can validate the ordering is "good".

Attempt at solving a suspicious ASSERT firing on AMD Win7.

Bug: angleproject:5198
Change-Id: I7750eff7f8d3aff397e7880f997ad86c9bf7d7a8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2490342
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>